I have not lived in the UK for nearly 12 years now and see no valid reason for doing so. In this time I have been ‘back’ about four times. Once for a project, once to renew my passport, and once for a wedding. In-between I have been in transit via Heathrow about four times.
Perhaps someone else has noticed this because at first I thought it was just me. Watching several BBC / UK TV channels a while back I noticed that people seemed to have developed lisps. The more I listen to any shows on TV, DVD, the Interwebtubenets, or radio this seems to be reinforced.
Check out this as one example here. Ignore the subject matter, it is the voice you should be paying attention to.