Only two minutes before you posted this, you'd already got 2 guitars. Are you thinking of getting another one? Is your family forming a rock group? ;-)
You're allowed �145 of gifts, souvenirs, etc, without having to pay tax.
The import duty on guitars is quite low (between 3.2% and 3.7%) but there's also VAT to pay.
I'll assume that the higher rate (3.7%) of import duty applies. I'll also assume that you've already used up your �145 allowance. Lastly, I'll assume an exchange rate of �1 = $2.
So, a guitar bought for $200 (�100) will attract import duty of just �3.70. VAT is charged on the purchase price and on the duty. So the VAT is 17�% of �103.70 = �18.15. So the total to be paid is �3.70 + �18.15 = �21.85. i.e. the total amount payable will be roughly 22% of the purchase price.
Chris