In reply to Philip:
I actually did some research on this sort of thing as part of my degree.
The maximum you can use for feed-in is 4kW as others have said, which amounts to a 16A breaker being added to your consumer unit. Any more than this, I assume, would mean additional inverters and cabling.
We have a 4kW system installed which provides 3.85kW maximum on a sunny day with no cloud cover whatsoever and no shading (shading makes a massive difference to output). On a really good day we make, maybe 25kWh, on an average day in July about 18, and on a shitty day in December maybe 3.
Taking this into account, if we say a GSHP (or any type of medium-sized HP) uses about 2-3kW constantly, during 24h it will use 48-72kWh. This means even on the sunniest day (when your house is lovely and toasty from the sun anyway) you will be paying out for the majority of the energy used. Of course you get payments from the government too, but that probably wouldn't cover it.
On that freezing cold rainy day in december when your panels are covered by snow and not producing anything, the GSHP will be drawing its maxiumum power, and you will be paying out for all of that!
Ontop of all of this, a heat pump requires a highly efficient heating system to be used. Underfloor is perfect because it has a massive surface area. The GSHP will only provide a low flow temp of about 40-50 degrees, so the bigger the heating surface the better it will work. Radiators (unless the are huge) won't cut it and heat will just be wasted.
You also have to consider that it will never be HOT in the house, you won't be able to crank up the heat quickly, it will just remain warm all of the time. You can do day/night temperatures though to save it heating on full whack all night.
Using a heat recovery system may also be beneficial, although still, thats more power being drawn from your already underpowered PV system.
Oh and finally, all of this is worthless if you still have normal haolgen bulbs throughout, if your loft is only marginally insulated etc... etc...
When we had ours installed we doubled the insulation in the loft (which was already twice the minimum requirement), insulated all of the walls, upgraded the old windows and replaced the majority of the lighting with low-energy or LED lamps. Its saved a fortune in electricity, and now I don't actually think we pay for our power in the summer months!
I hope this helps, and please don't hesitate to get in touch if you want more information