Sigh.....
The Chancellor is appointed by, and works for, the PRIME Minister. The incumbent Chancellors job, as for the rest of the Cabinet, is to deliver policy as laid down by the PRIME Minister based on the manifesto. They are not independent post holders with their own ideas, policies, plans and agendas.
Some people need to remember the other title on No. 10 Downing Street, and the PM's additional title; FIRST LORD OF THE TREASURY.
And BVRAAM, it was £15b over five years, not £15b per annum. The Chancellor has, allegedly as no-one actually knows, proposed a £1.9b increase for the current fiscal year only, in line with all other depts one year spending review. The same thing happened last year when the multi year comprehensive spending review was canned and replaced by a 1 year settlement, Defence in that case was given an additional £2b.
So the difference means that the increase will either be £3b per year for the next 5 years, or £1.9b for the next year only. Next year there will be a multi year comprehensive spending review, or another one year settlement.