There's a well known recumbent shop in Glasgow called Kinetics. If you haven't already been to visit, you should check it out and do some test rides. There's also Laid Back Bicycles in Edinburgh. Velovision Magazine has a long list of UK recumbent suppliers.

Pretty much any recumbent can carry a commuting load. Just fit a rear rack and get some panniers, or attach a basket on the rack. The main thing may be that you might want something with a slightly more upright and higher seat - more of a touring recumbent (you can of course commute on a low racer - people do. But as a beginner it may not be the best choice).

Your biggest problem may be finding a (new) 'bent for around 500 GBP. For that sort of money you could almost certainly find something decent second hand though. I know it's a long way from Glasgow, but you might contact Kevin Dunseath at DTek (just outside Cambridge) as he has a huge range of used machines. He has never had a web page, but try dtekhpvs (at) btconnect.com or call (01353) 648177. They are at:
3 Main St, Little Thetford, Ely, CB6 3HA. I never bought anything from him, but for a small fee he let me test ride around 20 different 'bents and it really helped narrow down what I was looking for.