I would let them know I was immersed in what I was doing if they continue, but...this may read surreal, does it not depend on the aesthetics on the 'stranger' - would you (male) return the serve to, say, a 24 year old female student the same way you would to a 42 year old mother of three (with or without the bairns in tow)? Or what about a lass...would the aesthetics of the stranger make your decision to return his serve (be it 'good morning', or anything else innocently).
Of course, most Brits still tend to hold on to the old school concept when it SUITS them nowadays. I admit myself, if a lass said 'morning' to me, I would return the serve (as I always have done)...but if she looked like, say Charlotte Church...to pick a name out of my wee head - then I would try to push greetings to conversation as swiftly as possible - but if she looked like the classic hen pecked soap-opera dearie, would keep words down to a minimum ('tis a male thang, I'm afraid!). And I suspect...be it on the Tube under London or in the queue of a shop in Norwich, we all group people and make presumptions, whatever our sex.
However, if someone is trying to engage you whilst you are doing a crossword or Sudo ku, then that is pure rudeness - but then again, would we snarl or smile depending on the aesthetics of the person (of opposite sex) who spoke.
Those 'total strangers' are also train commuters...and judging from the paranoia about something else altogether in London of late, could be you will be in situation when he/she may save your life - if of course you are being pestered almost to criminal levels, (by whatever sex) I'm sure...even in London...the rest of the 'strangers' in your carriage will be strangers no more.