I got invited multiple times by my surgery (plus I could get one privately) but I declined, figured I'd save it for someone in a higher risk group.
The effectiveness is quite good (current COVID vaccines are highly effective, in trials anyway), but depends on the strain, but also even if you still get flu, symptoms will likely be reduced and there is a lower risk of significant complications (which include death).
Herd immunity refers to something else, the proportion of a population that needs to have significant immunity to prevent effective transmission of an infectious agent (basically, this means it has nowhere to go).