Buying a road bike (potentially); advice needed please :)

I think you should consider either buying a bike now that can handle the Lands End trip which means spend a bit more money to get something that can, or use the mountain bike you now have to commute and save up money to get the better bike to do the trip, or buy a used road bike for around $250 US dollars and save up money to buy a nice road bike to the trip. Don't waste $650 to commute on then find out it isn't good enough to do the Lands End trip. Think ahead.

If you can afford it now I would be looking for a road touring bike or a cross bike, but I think the road touring bike would be more idea since it will have eyelets for panniers both front and rear instead of just rear.

Not sure what is available in the UK, but for the money the Kona Sutra is probably the best deal in a touring bike...at least in America. Read about it here: http://www.konaworld.com/sutra.cfm And this bike is only about $874 pounds.

Otherwise you should look at Ridgeback Expedition for $792 pounds, but no front pannier rack and not quite as well equipped component wise as the Kona. See it here: http://www.evanscycles.com/products/ridgeback/expedition-2014-touring-bike-ec059663

If I'm barking up the wrong tree in advice just holler back.

The good thing about these bikes, is they come with racks for rear pannier racks, and the Kona comes with both front and rear racks.

I forgot to come back to thank you. You gave me some stuff to think about. I bought a cheapish road bike in the end. Commuting on it is great. We didn't do the lands end trip this year but we did do coast to coast (west to east) which was a fantastic experience. Thanks for all the help way back in March!