Not sure of the full rate for London but from Norwich I started on £16775 and am now on just over £21k after 1 year, so that's the sort of climb to full rate after 2 years.
You will do 5 days at Stratford as corporate induction, it wont just be "your" group but anyone from any position who needs to go through it, so expect to see lots of people never again. After that almost everything is done at Norwich, the only exceptions are trips out but these all start at Norwich. Those trips include a visit to Colchester PSB, Anglia Control and perhaps a lever signal box (for my intake we went to Wymondham as it was on the way to Ely, where we went to see lots of example of different signalling infrastructure in one place) but now I assume you would go to somewhere on the Wherry lines. Also a lot of other small trips here and there.
In short if you are London based get used to a long commute!