Thanks for all replies.
Marie13 - I've done that a few times already and I tend to do it every 3 months or so. I feel very comfortable in the pool and I constantly do drills like no mask swim, regulator recovery, safe ascent, etc. However, I don't feel any pressure there as the pool is just 3 meters deep so it's always at the back of my mind that no matter what happens I can surface safely (never holding breath ofc) but I'm not so used to 15+ meters in open sea.
When you say “club” do you really mean a pool session run by a shop? A genuine club, a branch of BSAC, the SAA or ScotSAC, will be trying to get you diving in the U.K. to fill spaces on boats rather than trying to sell holidays. In London there are several large BSAC branches (eg
http://www.hellfins.com/, Clidive.org, Bermondsey (my branch)). The point of a club is to enable diving, get people together as buddies and generally have advice and information. If it tries to sell you stuff it is not a club but a shop. The club on the side of a shop is a common thing but a bit of a con often, providing upselling opportunities.
if you join an actual club they’d want you to do BSAC Sports Diver, learn to use a drysuit and get out on boats as often as possible. The have fees of the order of £150 to £300 per year which pays for pool hire, training kit maintenance and purchase and covers empty spaces on boats (we burned £300 that way on Sunday) or the cost of owning a boat,
in your specific case, of being apparently qualified but only having done shallow dives in easy conditions, the plan would be to do a few local lake dives as somewhere like Wraysbury or Stoney Cove and then get you on easy trips suitable for Ocean Divers - we regularly go to Pembrokeshire, St Abbs, and sometimes aim for shallow depths out of Brighton.
Clubs are specialists in taking non divers and making actual divers. There is more to diving than a plastic card. Because the relationship is a long term one, rather than here today gone tomorrow, and because the club needs to retain members it has to look after people and not scare them.
If you turn up at some commercial holiday resort they will try to fit you in with the dives that are happening. Those will be to whichever sites are popular and most spectacular, they might be deeper (see all the arguments on here about how deep a OW diver can go to) than comfortable for you and the result may be quite unpleasant if you are worried by the depth.
if you don’t want to join a club then maybe call Wraysbury and do a drysuit course and then some of their “new diver deal” guided dives. Just to get some time in, then maybe do AOW. Another option would be to do a bunch of one to one guided dives in Malta. Ring up dive centres and have a full and frank conversation.