I did research this topic some time ago and the reason so many cases of sarcoma have been reported in the US was, I think, largely put down to a particular type of rabies vaccine (vaccination against rabies is mandatory in the US unlike the UK). Unfortunately I can't remember what it was about this vaccine that was thought to be the causative agent. However, injection site cancers although rare, are not completely unknown in the UK, so it would make sense to inject in the leg for the reasons you mention.
As to whether its safe to give boosters annually, so long as your cats are healthy and have never had a bad reaction to vaccination then I wouldn't worry. At 9 years your cats are not really old anyway. If they had a compromised immune system or severe kidney failure then the risks would probably outweigh the benefits.
The other question of course is whether they NEED annual boosters. If you might have to put them into a cattery at any time then the answer is yes because they will insist on it since the product information sheets for the common vaccines say annual boosters must be given. In fact there is a lot of evidence now to suggest that every 2 or even 3 years may be sufficient for boosters but in order to change the product information sheets the manufacturers would have to submit supporting evidence to the licensing authorities. That process would be time consuming and expensive and, lets face it, they are unlikely to want to spend time and money requesting a change that, if approved, would actually lose them sales.
Edited because I got my risks and benefits the wrong way round.