looking at a selection of car hire broker websites, none of them will give you a drop-down box to enter a drop-off point in another country.
you could probably do this with any of the big named hire companies, (according to a travel writer in the times), but it is likely you will need to negotiate with them by phone. but to give you an idea -
with hertz, a 1-way hire from brussels to heathrow (using random dates) for an 5 seat saloon car will cost you �135 per day. the drop off fee for the 1-way hire is EUR655.00, which is more than 3 times the daily hire rate. With taxes and charges, the cost will be about �1150.00 (without fuel).