Don't knock Shawbury and the advice given by them. The ATCOs there are unfailingly helpful and pleasant to everyone, including those who require serveral attempts to get them to read back correctly - I've heard them often enough! They are GA friendly and more than willing to help keep you out of trouble. I would far rather take their advice that I might be about to infringe Birmingham's airspace and find myself a couple of miles further away than risk infringing it. I will ALWAYS talk to Shawbury but rarely to Birmingham, especially after my last attempt to do so, preferring to keep down below their airspace.
A decent IFR GPS will be accurate to well within say 10m, everywhere, virtually all the time
Which could easily have meant you were eight miles inside, instead of two miles outside, Birmingham's zone. I'll take Shawbury's advice any day.