The straw-man carbon sequestration process is to grow plants, cut them down, put them in a sealed box, and throw it in the sea in a subduction zone. Let's see if this is cheap enough to do on a personal level:
The convenient subduction zones are just east of the Caribbean, just south of the Java-Sumatra island chain and just east of the Philippines - there are also ones just west of British Columbia, just west of Chile and along the Aleutian islands, but those aren't on shipping routes from Britain to anywhere that ships normally go, and I suspect the 'just' means they're within territorial waters.
It looks as if a used twenty-foot shipping container costs about its weight in scrap iron, which is a few hundred pounds, is reasonably sealed for these purposes and holds eighteen tons (ah, bother, it's 30 cubic metres, so it would float, and if you make holes in it then creatures will come in and eat the compost which defeats the point of the sequestration.
I have the strong impression that the compost made from collected domestic compostable waste in Cambridge is essentially free; use of small bulldozer for a couple of hours to load eighteen tons of it into the container, use of lorry to transport container to Harwich. Container shipping is currently extraordinarily cheap (though maybe that's only to Shanghai via the Malacca Strait, and to get over a subduction zone you'd need to ship to Bali, Manila or Caracas).
The show-stopper is convincing the crew of the ship to load your container on the outside and to push it over the edge somewhere just south of the southern edge of Indonesian territorial waters; container ships don't have the cranes on them to move the containers.
What have I missed? Aside that it looks as if it would cost about a thousand pounds to transfer twenty tons of compost to the bottom of the Philippine Sea, whilst www.puretrust.org.uk will buy and retire ETS carbon at £13 the ton.
Maybe if you bought an exceedingly rust-bucket container ship and a medium-sized escape boat, and sunk the whole ship and five hundred containers as a unit ... claiming it on the insurance afterwards would be wrong.