Hi Oliver
First of all you need to determine your budget, make sure that your towing vehicle is capable of towing a large t/a caravan safely and legally and then perhaps a look round one of the forthcoming Caravan exhibitions either at Manchester of Birmingham would give some idea of models, layouts etc all in the same place.
Click Here for details of the shows.
Practical Caravan magazine has the details of manufacturers, layouts, number of berths etc in it's monthly caravan buyers guide (Page 112)
Both shows and the magazine buyers guide feature new caravans but you will get some idea of layouts and manufacturers from them.
This website hosts a sales site where used caravans are offered for sale by both caravan dealers and private vendors.
Enter the details of what you are looking for, price, distance from your location etc and suitable models will be shown along with pictures and contact details.
To access the Buyers Guide
Click Here
Please feel free to ask questions on this forum, better to ask than to face the possibilities of expensive mistakes later.