Actually, it costs Deutsche Telecom £625m
You said "That starts with Orange chucking in £1.25bn to make up the difference in value between the two companies" - that's not right, it starts with Orange chucking in £1.25bn of *debt* - i.e. the new venture starts off owing Orange £1.25bn. And then Deutsche Telecom throw in £625m of real cash, which goes straight out to Orange to balance the debt to the parent companies.
So in effect Deutsche Telecom are paying Orange £625m to take on T-Mobile UK.