The gardening association locally sells
Kings Seeds which are good value for money.
I've bought organic stuff
from here and found that they are good value.
You could also try Mr Fothergills - the instructions on the packets are very good and make up for the extra price.
If you are looking to buy on a budget you could start at
Alan Romans - the packaging comes I believe with no instructions and there are possibly not quite as many seeds in the packet but the prices are good and I've heard a lot of good things said about the seeds germinating well. Limited choice but not bad for the average allotment.