I've done this several times. The duty (on acoustic guitars) is 3.2% - not too bad. You'll then pay VAT on a) the original price plus b) any carriage charges incurred plus c) the duty. Yes, you pay tax on the tax...

Twice I've arrived at Manchester from Newark with instruments that I always intended to import legally. On both occasions I had to wait for a suitably qualified customs person to arrive and do the paperwork. It has to be said that I could easily have walked through on both occasions.

I've had guitars delivered by Fedex a few times. There's a bit of a lottery with duty...most Fedex packages get caught when they arrive at Stansted but not all. I know someone who had a very expensive custom Martin imported by Fedex that wasn't inspected. That was a real result, but not to be relied on I fear!

If you buy wisely you'll save a packet even with all the extras. Good luck!