If its your guitar - and is not covered with swing tickets, sales receipts, plastic pick guard cover etc and you are bringing it to use on your visit, there should be no problems - its just luggage, as noted. If you subsequently decide to leave it in the UK is nobody's business but yours. Otherwise, if you are stopped in the green channel with something that looks like an import its very likely a surcharge will be levied, plus the import duty and VAT.
If it's for your "personal use" while in the UK there should be no import duty. As mentioned above, I wouldn't leave the tags, etc.on if it's a new guitar though.
If you declare it you will have to pay import duty and VAT, somewhere in the region of 22% for an electric guitar.
I've brought about 3 guitars back from holidays to the States, and always walked through nothing to declare. I've bought plenty of gear at rip-off Britain prices, so I think saving a couple hundred quid on a guitar ever now and again isn't that immoral.