I've asked a couple of kayak clubs in the UK if they could respond to your questions. Hopefully we'll have some answers soon.
In the States; tide, current and navigational charts are available through stores or chandlers that cover both recreational and commercial boating. As well we are able to buy direct from the source if there is not a store near by. I would think that if you have a commercial boat type of chandler in your area that they would carry the British Admiralty Charts.
Sea Kayaker Magazine