Ah yeah sorry, so that gov.uk you linked to actually says effectiveness between 15 and 21 days is 89% and the 52% is efficacy between dose 1 and 2 - short-term protection from the first dose is 'very high from day 10'.
Still won't be able to go round for a cuppa for a bit though I think.
The 52% often quoted includes the early period when it does nothing. The effectiveness after 15 days is estimated at 89%.
They also reckon the Oxford vaccine is 73% effective 22 days after one dose.