The most important thing that anyone learning to play guitar within the context of traditional music can do is play with other people. Learn the basics - the most essential chords for common keys and basic strumming or fingerpicking technique - however you can. Find a willing friend, pay a teacher, whatever. But as soon as you have the bare basics down get out of the house! Go to every session, music party or festival you can get to and play. The more you can immerse yourself in the music, the quicker you'll learn.

Listen to the techniques other players are using and try to leave each session having learned something new. And concentrate on timing, timing and timing.

And, if you're already attending sessions with your fiddle force yourself to leave the fiddle at home and just take the guitar. I accompany three fiddlers on a semi-regular basis and they're all decent guitar players as well. But they'd all be a lot better if they'd leave the fiddles at home occasionally and just play guitar.

As for instrument recommendations... From other threads it sounds like Yamaha guitars are readily available in the UK. You can't go wrong with one of them. They're relatively inexpensive, sound good for the money, and come in variety of body sizes. Get a steel-string model, put silk & steel strings on it until your callouses build up to guitar-player thickness and change to bronze or phosphor bronze strings as soon as they're reasonably comfortable. (Silk & steels don't have enough punch for many accompaniment situations.)

Bruce