If you bring something in from outside of the UK then you take the item price, then add the shipping. Then add VAT to all of that, and then add the import duty for the thing. Import varies depending on what class of item it is. But guitars are about 3% ish.
Then when it lands there is a load of HMRC paperwork to do - however the UK courier will do that for you and charge you between £8 and £15 for that depending on item value. A lot of people complain about the admin fee but TBH it is a godsend. Before the couriers were able to do it for you, you'd get a notification that your item had arrived in the UK and forms would be sent to you to complete. You'd then send them back in the post and then in about 3 weeks they would be looked at. Then they would send you a bill to pay. You'd pay it, and then in another 2 weeks they'd finally release the item to be delivered to you. The item would sit in a customs sealed warehouse for over a month. These days you get an email, pay it over the internet and it only adds a day or 2 to the process. And you never get the forms being returned because you didn't do them properly - the couriers have people churning them out all day.
So whatever item you want, assume that to get it into your hands will cost 25% over the item sale price and you won't be far wrong.
Unless it's something you can't buy in the UK the savings are often minimal and of course if something goes wrong with it you are at the mercy of the seller country laws and regulations. Many of them don't offer warranties once an item is exported, or you'll have to pay return shipping and never get that back.
That being said, I've had a few things from Ishibashi in Japan and they are superb. Amazing service and excellent packing.