Be VERY careful where you buy if intending to transit to domestic at LHR (or elsewhere similar hubs). ‘Security’ has a list of places they trust, and EVEN in sealed bags duty free may not be allowed to transit to EDi or anywhere. You can take your ‘risk’ onto a London bus or Heathrow Express, but not on to Scotland.
Any plausible risk assessment would place VERY high emphasis on suspect packages being taken onto buses or tube (because they’ve been targeted, recently and effectively). So t’s about the money, not the passenger and their safety.