SCUBA trip with dive club recommendations European mainland

[Cumulo-Nimbus]

Oh, now that you mention it: Denmark has great drift dives in Gammel Ålbo, it's mostly beach or jetty entry and you can expect the current going from eezypeezy to whipping in any direction .

For a fee the owner will bring your group by boat to the Lillebelt Bridge at Middlefart, to change the dive sites a little.
That's mostly shallow one-way dives, depending on current speed you could hop in, drift along the wall running parallel to the beach at about 8-12 Mtrs and exit at a certain point (there's a big chain lying across the bottom) to walk back a bit - repeat
we once did about three dives on one tank, about 10-15 mins each.

 

[Rich1280]

I'd second Plymouth as an option. You can ferry from Hook of Holland to Harwich, and then its about 330mi / 530km drive to Plymouth, about 6-7hrs comfortable driving if you stop for lunch and things. In Deep can easily cope with a club, we use them for our club trips and there are a number of accommodation options for you. Boat size is usually 12 divers max but can be multiples, or sections of, and they might run a mixed boat for a tech trip with your divers and others to full it so you don't need to charter the whole boat. That can work quite well. Here's a list of some of the dive sites on a club webpage:
Local wrecks and Reefs around Plymouth | Plymouth Sound Divers

For a Scapa trip if you fancied that, you could ferry overnight Rotterdam to Hull, then its 530mi / 850km up to Scrabster to get a short ferry to Orkney. That's doable in a day, then ferry will drop you right by the dive boats you'd do the liveaboard off. The other option is the Aberdeen ferry, which is 400mi / 640km from Hull and a very pleasant drive through remote Scotland at the top end. That would drop you in Kirkwall rather than Stromness so about 20 minutes drive to the dive boats. Maybe need to line the ferries up a bit to see what works best, but both easily doable and a great week,

If you were a little flexible on your package and wanted to book a hotel and diving as separates but all in close proximity, Portland has options as well but would take a little more work. I can talk you through that if you like, drop me a PM, I've just organised one for my club but we did one boat per day for all similar depths.
Come to the UK, we have great diving to share!
Rich