The advice is we will only be offered Pfizer or Moderna if available, and if it won't cause delays. It is not guaranteed.
However...
The booking website is being redesigned to allow those under 40 to request a Pfizer or Moderna jab specifically, and vaccines minister Nadhim Zahawi said the NHS was “having to deal with a much more complex set of deployment challenges and doing it brilliantly”.
If no one is seeing this change, it isn't in place yet. The website still says over 40s, so it must not have been updated yet.