UK is getting this vaccine first because MHRA approved it before EMA did (and any EU country is allowed to use this shortcut that MHRA is so far the only one to have used), not because we have a deal to get it first.
"I'm not convinced that if [the UK] had been part of that European buying programme they'd have had quite the speed and flexibility to do it." -- Hugo Fry. (Pharma head, who happens to be a remain supporter.)