In the UK, where do I buy a “pocket door” kit from

A pocket door is a door that slides into the wall as opposed to opening in or out as a regular door would.

A good comparison would be the sliding doors as most mega-marts or large stores that slide open when you walk towards them or if you're a fan of "Star Trek: The Next Generation". The doors throughout the ship are "pocket doors" (except our pocket doors don't make the neat hissing sound when the open).

There are a number of pocket door appraches. Usually is makes sense to buy a frame kit such as this one

There are many grades and sizes, some of which are meant for heavy doors. You do need to make sure that the thickness of your doors do not exceed the limits of the kits. If they do, you may be able to buy parts and create your own combination that will suit. Also, there are standard heights, which could be adjusted for (e.g., with bottom blocking) if your doors are especially tall.

There are other brands that may provide additional features, such as this one which has a color matched header insert, and this one which can be used in thicker walls , and perhaps with thicker doors.