Giles,
One does not have to be near the boundary to interfere with an adjacent FIR. An aircraft at 2000ft near Birmingham will be picked up by an aircraft well into the Paris FIR at FL350 and the Shannon FIR at much lower levels.
If you want to work out where a practice can be made without being received by aircraft in an adjacent ACC's airspace try the following;
On the basis that except for a very few mostly UHF equipped aircraft, the highest cruise in current use is FL500ish (50000ft)
The reception range at 50,000ft is about 270 miles when the transmitter is at sea level.
Get out your chart and draw an area that is at least 270 miles from all FIRs adjacent to the UK..........At a guess it might leave you with sone airspace in Scotland where you can go and practice without interfering with other countries emergency traffic............and we are dealing with a transmitter at sea level where D+D won't even pick up the call.
Regards,
DFC