DS, MRNA vaccines are the preferred vaccines for booster shots anyway and what they will be using in UK so AZ not relevant in that regards.
There is no issue in having had AZ and then an MRNA booster, there was some talk/theories but no studies that AZ with an MRNA booster might be an advantage?
Being over 50 and having had a history of health issues I will still be pretty cautious until I can get my hands on a booster.
Who is in line to get it, which vaccines are being used and how will they be distributed
www.theguardian.com