MrsPear cannot be in UK, as the immunisations here are not and never have been single strain. There are three of four, depending on your age and which year.
But yes, everyone knows it is based on a forecast of dominant circulating strains (continuously updated - it set back northern hemisphere manufacturing for a month or so this year).
Yes, sometimes they could have made a different and (with benefit of hindsight) 'better' selection. But most years it's not wrong. And every year there has been a reduction in expected case numbers.
Yes, strains not included will also be circulating, and yes you might get one of those. The aim of the immunisation is not to stop all flu. It is to reduce it sharply.
I wouidn't criticise someone for not getting the flu jab, even for reasons which are non-applicable in UK.
But wouid add, OP, that your GP should be able to offer a catch up becabecause they should have a supply of sniff for all those who did not get it at school (eg under 3s, HEdders, those ill and unable to receive it in school - you can't give it to DC with very snotty noses as it won't 'take'