Jimbo
Another possible option is that it used to be that all smaller aerodromes could submit flight plans through their relevant regional parent a/d (Heathrow, Manchester, etc), often via fax. It's a while since I worked at EGLL, and I know the system has changed since I left, but the details of any parent/child a/d setup that may exist should be in the AIP; if you have trouble finding them, give EGLL FBU or AIS a call and ask them.
The only trouble with that arrangement is that the parent a/d gets all the ACKs, etc, so you need to have a solid procedure to cover that, but if you're using the structured editor to check any plans first, then that should be fairly acheivable.
I'll try and find out a bit more soon, but things are a bit busy at the moment, so may not be possible.
FD