Interesting conundrum. I would expect that checking 1 bag through from BCN and then checking in a 2nd bag at LHR would cause all sorts of problems. The alternative would be to check in at BCN only as far as LHR, pick up your baggage, collect 2nd suitcase and then check in again. But that 2nd routine is also a bit fraught if there are any delays.
In either case, what are you going to say when asked at checkin at LHR "Did you pack this yourself? Have you been given anything to take on the plane?" If you're honest I would strongly reccomend that nothing in the bag collected at LHR is wrapped. In fact, thinking about it, if you try and add the 2nd bag at LHR it's going to be blindingly obvious that you did NOT pack it with all the attendant problems that will cause.