Montecatini Terme is a well known spa resort on the road from Florence to
Lucca. It's of course not a fancy place, just hotels, spas and parks, but
from there you can easily take many day trips to Lucca's villas or Pisa.
The Gatwick express takes 45 minutes to Victoria station: just across the
station you can board a narrated bus tour around London. The ticket allows
you to hop-on and off the bus anywhere for sightseeing or lunch, and then
board the next one. The bus runs every 15 or 20 minutes through the day,
the ticket should be around 5 pounds.
Have a nice trip both of you.