Maybe check out this thread:
Also today ‘This week in Virology’ has just done a Q&A on covid including boosters, and tomorrow will be talking about Covid and immunity with Paul Offit who is a virologist who advises the US government.
You can at least book an appointment if you haven’t already done so (though I personally decided I wouldn’t queue at a walk in one given the level of infection at the moment - I’ve gone for a place where it is by appointment slot only, to reduce the wait time and hence number of people I’m in contact with!) You can cancel if you change your mind - it will be snapped up by others. Is your GP surgery offering them?