Hi, I'm Larry from Devon, UK.

Hi Larry
Wow that is very surprising.
Where was the compost obtained?

I would first put a plastic bag around your geiger counter to avoid it getting contaminated.

Put some rubber gloves and a dustmask on. Then put your flowerpot, compost and plant into some sort of large shallow container or cardboard box and see if you can determine exactly what is radioactive. Is it the pot? Is it the soil? Is it a source hidden in the soil? Maybe even video the process.

8000CPM would be some seriously contaminated soil. If you find an exact source, handle with tongs, check it for CPM, isolate it and store it away from people.

Then tidy it all up and seal in a bag, inside a container.

Who would deal with it? I'm not entirely sure.

But now you have identified it you can make it safe and check to see if the compost was used anywhere else.

What everyone else said.

If you wanted to, then as mw0uzo said, spread it out on something disposable. A couple of black bin liners or something similar. Split it into two piles and measure both. Discard the pile with no reading and then divide the 'hot' pile again into two. Keep on doing so until you find the source. Seal a plastic bag around your counter and discard the bag when you have finished. Do wear gloves and at least a dust mask. A respirator would be better. Not knowing what the source is you should take precautions. Maybe wear two pairs of latex gloves (in case one breaks) and also old clothes that you could throw away if they became contaminated, or better still a disposable waxed paper suit. I suspect there is little danger from the radiation itself, but you do not want to breathe in or ingest anything; that is where the danger lies. After you have finished then check yourself with the counter, especially the dust mask/respirator. Anything that is contaminated needs to be sealed up in a bag and disposed of properly - do not burn them as this can release particles into the air and they may travel on the wind.

If you want to report it I would suggest the UK environment agency. There is a web page here with some details and may help you: https://www.gov.uk/guidance/monitoring-radioactivity.

If it were me, I would get suited up, gloves, respirator, and go looking for the source. But what to do with it then? Keep it in a lead box as a souvenir?