The cheapest are the usual crowd:
www.bes.ltd.uk
www.screwfix.com

plus, you can try eBay. Do a search on this term (speedfit, speed fit,
johnguest, john guest) and you'll get a long list of items for sale.
It's usually the cheapest way to get coils of pipe and bags of
fittings, but not so good if you want a specific single fitting. I've
used this route with some success.

If you only want one or two things, it's better to go to B&Q Warehouse
or Wickes, but the fittings are about 2x the other sources.

You can also get pushfit radiator valves, if you want.

BTW, for short runs (less than 3m), I'd suggest getting the straight
lengths of pipe rather than coiled. The coiled speedfit pipe doesn't
like being straightened (especially the 22mm stuff).

Always remember to cut the pipe square and clean using the proper tool,
and use the right insert for the plastic pipe and fitting (speedfit has
two different inserts for pushfit and compression).

-Antony