I understand that in my borough (Lambeth) it due to the poor uptake rate of children receiving the booster between 3-5yrs (and therefore being fully protected).
If you read the 7 page doc (link below found from a quick google) it sounds like this might also be the issue in Wandsworth (relevant info is roughly page 3), hence they advise the booster by 2yrs instead of pre-school as per your red book.
http://ww3.wandsworth.gov.uk/committ/(S(mmnhu4ih3qozyo45zsrk0ma4))/documents/s13587/Paper%20No.11-299.pdf
On the NHS website it says that the booster can be given as soon as 3-months after the first jab if necessary.
I too was quite surprised to receive a letter inviting my then 18-month old for her booster.
I asked my old health visitor in Kent and a GP friend and they both said unless there was an outbreak then I could wait until she was over 3, which is what I've done.
She reacted so badly to the first MMR at 13-months I was loathed to put her through the same thing only 5-months later.
Hope this is helpful (I'm not a medical professional
)