As others have said, you don't (sadly) need any form of qualification to use the boat at sea. However, you also ask about inland waterways, and here it is slightly different. On canals or any other navigation managed by British Waterways, you will need a sticker for the boat, and need to have passed a safety inspection if you're on the water for more than 30 days. Check the BW website for further details.
The Thames is managed by Port of London Authority (tidal) and Environment Agency (fluvial). Again, details of appropriate schemes on the websites.
You do need a licence for a VHF radio, both for the operator and the radio. For operator, do a one day course via the RYA (www.rya.org.uk
) and for the radio, apply online to Ofcom, which is free.
For SSR registration, it is advantageous if using the boat abroad. Go to https://mcanet.mcga.gov.uk/ssr/ssr/
for further info.
Loads of insurers about, but check very carefully any foreign usage limits.