Depends on where they are going in the UK, take a raincoat either way.
As HM said if they are in any of the big city's there will be a John Lewis which will have Kent (MWF) and a range of Kent brushes they also have maybe one other soap but mostly else it is Kings of shaves or nothing special that you could not readily get in France.
Supermarkets will have Palmolive sticks and cream and maybe Erasmic and sometime a cheap brush.
In some of the malls there are Molton Brown outlets (again easy to get in France), if in Edinburgh or Glasgow Arran Aromatics is worth a visit <!-- m --><a class="postlink" href="http://www.arran-aromatics.co.uk/index.asp" onclick="window.open(this.href);return false;">http://www.arran-aromatics.co.uk/index.asp</a><!-- m --> not tried their soap ina longs time but it used to be really nice, Mantic was a fan but ut has had a formula change. Not cheap but they may even find something they like.
They may see markets with soapmakers, many of these will sell what they claim to be shaving soap but avoid that unless they know it is good stuff, a lot if it is nice handsoap but no good for shaving.
Where are they going and I can have a hunt around that area one the web for you?