All 3 of mine have had singles, eldest has had all 6 (initial and boosters) - the other 2 are up to date but will not, as it stands, be able to get Mumps (middle one needs booster, youngest has had none).
There is currently no single Mumps vaccine available anywhere, worldwide, as the patent is held by Merck who are currently not producing it (thanks UK DOH (my friend works there)).
I personally am not worried too much about this at the moment as it is rare and not as risky as say Measles to get as a child - I had it and a lot of case are asymptomatic / risk of complications is not as high as Measles (in my educated opinion). The (unacceptable) risk, for me, comes with getting it at or after puberty (for boys)
I plan to wait until one of the following:
a) Merck are pressurised into restarting production of single Mumps vaccinations
b) They release the patent to someone else to do it
c) My children who haven't had it get to 10/11 at which time I will get an antibody test for Mumps immunity (they may have had it in the meantime) and may consider giving them the MMR at this point if they're not (less risky giving it to an 11y old than a 1y old imo).
We used to use a clinic in Croydon that has gone out of business (or been persuaded to if you believe the rumours). We now use SurreyGO in Guildford who are great, if pricey.