When looking for a Wetsuit you should consider what time of year you are wanting to use it. For example if your looking at using it in the months of November to April in UK waters then the temperature will range from 15c down as low as 5c. You will then need a Winter Wetsuit which will be a 5/4/3 in thickness, and what this actually means is the body is 5mm then a panel from the body to the arms of 4mm and arms of 3mm. All winter Wetsuits will be glued and blind-stitched which means on every seam they have been stitched then the two pieces of material glued together to form a bond which wont allow water to penetrate the seam. You should consider your budget as generally the more you spend the more features you will get on the Wetsuits making it warmer such as thermal lining and air neoprene. The wetsuit will also be far more flexible and again the more you spend the more flexible it becomes making it fit better to your body which will keep you warmer. Good makes to consider are O'Neill Wetsuits, Rip Curl Wetsuits, Billabong Wetsuits, Xcel Wetsuits to name a few.
If you are looking at a summer / spring wetsuit which are generally used from May to October then these are 3/2 wetsuits. Meaning 3mm on the body and 2mm on the arms. The same rule applies in terms of cost - the more you spend the more flexible and warmer the wetsuit will be.
Hope this helps and for the best range check out www.wetsuitcentre.co.uk