Sorry for pic quality. They are REALLY small. I have a vid but won't let me upload.

Hmmm.... don't think the are. I mean, do we even have mosquitos in the UK? I didn't think so....

It's really hard to tell from the photo, but they really do look like skeeters. Can you capture one and put it in a shallow dish to force it to pose for a picture? The photo you've got looks like mosquitoes (or related aquatic fly) but it's impossible to be sure from the image posted. If you keep them, put a screen on the container so you can verify what you get.

If they do look like mosquitoes and you don't normally have them in your area, see if there is a local agency or public health department that is interested. Climate change is making things...let's just say interesting when it comes to new species.

Those are some sort of insect larvae. They look like mosquitoes.

I'd say that they are absolutely mosquito larvae....yes, mosquitoes exist in the UK! (Which I'm sure you know by know if that tub is still acting like a breeding ground, lol.)

Tadpoles are waaaaay bigger than mosquito larvae.

Mosquito larvae. Yes, the UK has mosquitoes - 20 or so species.

Here's the control methods - starting with not letting tubs of rain water sit around and turn into nurseries.

https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/577340/Mosquito_control_factsheet.pdf

The tub has long gone, water and all. We've not long since moved into this house and are still clearing a few things up. AKA Tubs of rainwater, tons of weeds. We're not actively trying to breed mosquitos!