Edexcel exam board A Level Bio B Photosynthesis Req Practical

it's the a level version where you measure the volume of gas in an expensive bit of kit.

I tried petsathome it has bare shelves, and https://plantsalive.co.uk/ has shut down.

there is some on ebay, might try that.
Measuring the volume of gas means you can be more flexible with the pondweed than having to count bubbles (it doesn't matter if they are too small and fast to be counted individually) so pretty much anything that gives off bubbles from the cut end works. We also use hornwort from my pond.
Do you have any aquatics shops nearby? We have a garden centre that has a pet/aquatics section. There is not always much choice though.
If you can send me your address somehow I'll post you some Hornwort, when it starts bubbling in the right light it goes too fast to count anyway.
Got it, I'll sort some out tonight and post it to you tomorrow, one thing I've noticed about it is that it needs a bit f lead foil to weigh it down so the cut end of the stem is below the water.
Posted the pondweed today, should be with you tomorrow, I usually keep it in tap water that has been allowed to stand overnight, so a large tub or plastic food box can be used, it will be okay indoors for a while if sighted on by a window that gets plenty of light. There may be some critters swimming about in the box I've packed the weed in. We just use LED household lamps for our experiments but if can get hold of a Vaxer LED from Ikea the bubbles are an almost continuous stream, I use a 1% sodium bicarbonate solution in water for the experiment.