Best Glue available in the UK

Any recommendations for the best glue (available in the UK)or fixing neck?

You can get the standard Titebond red label glue here in the UK. It' dries just a bit harder than normal PVA glue, but like PVA, gives a bond stronger than the wood itself, so you don't need anything stronger. Used by most of the guitar manufacturers. Titebond also do a liquid hide glue, which has a longer working period but takes a bit longer to dry. It also has quite a short shelf life (about 6 months), so buy it just before use if you go that route. You can also use normal hide glue - but that requires mixing glue beads with water and heating it up - so is a bit messy.

Gibson use Titebond for most of their guitar necks, but use hide glue for the vintage re-issues (because that's what they used back in the '50s). Hide glue is physically harder that standard Titebond once both are dry, so supposedly transmits body vibrations a bit better, but I wouldn't worry too much about it.

The benefit of both regular Titebond and hide glue is that you can use steam or heat to break the joint e.g. if you find you've set the neck at the wrong angle. It's not that easy, but can be done. Other glues (like Titebond II and III, waterproof PVAs, epoxy and Gorilla glue) can't be steamed apart, so if there is an issue, there's very little you can do about it.

For a rebuild I did when we lived in Jersey, I used Cascamite - on the strength of a carpenters recommendation, and because he gave his pot to use!! - and was very pleased indeed with the results. However, I wasn't setting a neck into a body; I'm pretty sure that once Cascamite has set, it won't let go at all so Simons' suggestion of Titebond would seem wise!

Cascamite is a Urea Formaldehyde resin glue. It's certainly waterproof when dry, so not good if you might ever want to take a joint apart.