What you say is correct in principle, but ignores the important fact that practical car engines are horribly inefficient, and their effeciency changes quite a bit over the range of speed and power required to move the car. Note that this is the point of transmissions. At best they don't loose any power, but they make the overall process more efficient by allowing the gasoline engine to operate at a more efficient point.
In one way, you can look at a hybrid as having a wide-ranging finely adjustable transmission, but there's more to it than that. The efficiency of a gasoline engine is in part related to what fraction of peak power it must put out. If the gas engine is the only mechanical output in the car, then it must be sized to supply peak power. However, most of the time much less than peak power is needed, so the engine often runs at a inefficient point.
With a electric motor available to fill in the when peak power is demanded, the gas engine can be sized smaller and it is easier to make it more efficient over most of the normal operating range. It also allows for the option of not using the gas engine at all at very low power levels where it would be very inefficient. Instead it can effectively be run in bursts of more efficient operation. For example, if the gas engine is 3% efficient at 500 W, but 6% efficient at 1 kW, then you're better off running it at 1 kW half the time instead of at 500 W all the time. With a hybrid, you have this option. With just a gas engine, it's stuck having to produce whatever power is demanded at the moment, regardless of how efficient that is.
I have a Honda Civic hybrid, and I can tell you this stuff really works. I routinely get 50 miles/gallon minimum on the highway, often substantially more. The engine is physically small for the size car, and it has been specially designed to be easily shut down and restarted. Going down a hill, even at highway speeds, the engine often turns off. If the hill is steep enough, the motor is run as a generator and charges the battery. When I get to the bottom of the hill, I can see that for a little while the control system uses the electric motor to keep the car going at the set speed (this is all with cruise control engaged), then eventually gives up and switches on the gas engine. I can feel a slight klunk when that happens, and the charge indicator goes abruptly from discharge to charge.
In theory, in an over-simplified building physics model of your home's thermodynamics, yes, all of the wasted energy from inefficient devices goes to providing useful heat to the home, so increasing their efficiency will increase the heating you need from the space-heating system.
But in practice, no, your heating costs will not increase. Not unless you choose to turn up your heating.
At least, in general, that's how it goes.
The thing is, that incandescent lights tend to add heat to places where it's not particularly useful: the centre of the ceiling of a room.
For a combination of reasons, that's rarely useful heat.
Particularly in older UK housing stock, rooms have high ceilings with thermal stratification, which means a layer of warm air stays stuck at the top of the room. Tha layer is higher than thermostats and radiatior TRVs, so changing its temperature won't change the behaviour of the automatic heating controls.
What will happen is that the void between floors, or the loft, will get very slightly warmer. The void between floors is typically ventilated to the outside with air bricks, so your incandescent lights will warm up the outside air slightly.
The temperature field in most rooms in UK housing is pretty steep, with floors being a few Kelvin cooler than ceilings. The bigger the difference between the temperature at your feet and at your head, the less comfortable you are. The heat from lighting tends to make that temperature difference bigger, thus decreasing your thermal comfort.
and although you referred to peak summer weeks, I'm guessing your heating is turned off for a lot more of the year than that: May to September or so would be fairly typical in the UK.
To bear the theory out, see Brunner et al, who looked at heating bills in dwellings which had incandescent light bulbs, and those that had more efficient compact fluorescents (CFL). They found no support for the hypothesis that CFLs cause increased heating costs.
Best Answer
To turn thermal energy into useful work completely one would need a thermal bath at the temperature of absolute zero. This is explicitly forbidden by the third law of thermodynamics. The best one can do is given by the efficiency of the (theoretical) Carnot cycle: http://en.wikipedia.org/wiki/Carnot_cycle. Th efficiency of the Carnot cycle only depends on the ratio of the temperatures of the cold and the hot thermal baths that a cyclical thermodynamic machine has access to:
$\eta= 1 - T_{cold}/T_{hot}$.
As you can see from the formula, if $T_{cold}=0$, then the efficiency would be equal to one, i.e. all thermal energy would be converted. That, as we said, is forbidden, because of $T_{cold}>0$. On the other hand, if $T_{cold} =T_{hot}$, then the efficiency of any thermal machine is zero, i.e. one can't extract any useful energy from just one temperature bath.
Practical efficiencies that can be reached with real thermal machines range up to 60% (in combined cycle natural gas plants, I believe), but it becomes increasingly more expensive to improve efficiencies, so at some point the cost of the improvement is higher than the cost of the lost energy, at which point economics sets a limit to energy efficiency. A better way to use the lost heat is for heating purposes. Combining a small power plant with the heating systems of buildings makes almost 100% use of the energy in the fuels that are being burned in the power plant. These cogeneration facilities (named so because they produce electricity and useful heat) are playing an increasingly larger role in energy efficiency improvements.