You give them a LICENSE to use your image, NEVER a copyright, unless they're willing to pay through the nose for it.
Ask them how they want to use the image. Is it for web only? Do they plan to use it to sell products? Is it for an event or something they're hosting?
One year license to use an image usually (dependent on the use) is generally $500.00 per image. Not sure how much that translates into pounds, but that's taking into consideration any revenue they may make off of your image. If they balk at that, they might want to be reminded (nicely, of course) that if they're using it for monetary gain, the $500.00 investment usually pays for itself in products sold. People generally are visual creatures, and if they see something they like, they buy it. Besides, you have your own equipment to upkeep, bills to pay, and I'm sure you'd like to eat too.
The fee will help with that, and really should be factored into everything you do, sales wise.
If they want to purchase full rights, it's a whole other ballgame. You have to consider that you're not going to be making money off of this image by re-selling it or licensing it, and must take into account the lost revenue. You'd have to decide how much this image is worth to you. I've seen full copyrights sold for an image (meaning they can use it how they see fit and wouldn't have to buy another license from you) going from anywhere from 3k-5k. The downside to this, is that you won't be able to use the image, at all.
BTW, 55 pounds sounds pretty low. Maybe they're just testing if they can get the image off of you for cheap.