Not certain in this case, but when supersonic is authorised over land it's sometimes for just for one of the two QRA jets. That allows one of the jets to get into position quickly, whilst the other continues at subsonic speeds and arrives a few minutes later, but with a lot more fuel remaining. So it's possible you heard the sonic boom from one jet, and then the 'normal' noise from the second jet.
I did note that Brize threw a tanker up pretty quickly when the jets were scrambled
They always launch the "TANSOR" for a live QRA shout, assuming there isn't already a tanker airborne on training mission which can be diverted for the higher priority mission. They usually launch TANSOR for a practice shout too. It's quite often ZZ336, which is the specially painted "Boris 1" jet, because that sits on the ground waiting for its next VIP tasking, and so is 'available' for QRA shouts, whereas the others could be scheduled to go to Cyprus, the Falklands, or wherever.
Standard QRA procedure is to launch two Typhoons from either Coningsby (QRA-South) or Lossiemouth (QRA-North) and one Tanker (Tansor) from Brize. The tanker heads off to a suitable overwater refuelling area waiting for the Typhoons to show up. The two Typhoons then take turns shadowing the threat whilst the other goes off to the tanker for a top up. A single tanker can keep two Typhoons in the air for in excess of 10 hours if required.
Once the threat is over one Typhoon lands to be re-armed, serviced, refuelled, flush the loos, reload the egg sarnies etc, whilst the other stays airborne and alert with the tanker. Once the first jet is ready to resume QRA alert in it's shelter the second jet lands and is serviced in turn. This way there is always at least one armed jet ready to deal with a second 'threat' should there be two in close succession.
Finally, once the two QRA jets are back in their shelters and ready to go again if required, the Tanker pootles back to Brize for tea and medals.
The RAF/MOD never make it clear what the procedure is if more than 4 fighters (2 in Scotland, 2 in Lincolnsausageshire) are needed. Supposition is that the first 4 QRA jets are available in 15 minutes (typically more like 5 minutes) and another 4 can be generated in 30 minutes-1 hour. How that works in reality hasn't been tested recently though AFAIK. Can't see there being 8 pilots on duty on a bank holiday Sunday for instance.