According to press reports, that's because of their remit.
Meanwhile the rest of the world, particularly in Europe, the Far East and South America are catching up with the UK in terms of vaccination and in some cases overtaking us. We need to organise the booster jabs and vaccination for > 12 year olds asap.
Boosters, if they are as effective as claimed, could have a very significant impact in the population who receive them. I am very keen to get a 3rd jab, and for my elderly parents to get them too. That way, I can forget about Covid as a personal risk for months to come, and get through this Winter doing normal things.
If we don't proceed with the next obvious stages in vaccination then the situation will deteriorate gradually. We all know this which is presumably why it will happen only we are falling behind the curve now
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That chart is a proxy for advanced societies and it is noticeable that USA, the UK and Germany are not doing so well.