10000 pounds in 1812 currency what would be today's british equivalent?

For 1812 multiply by 80 or slightly more to get a modern equivalent, but please note that some things were much cheaper in real terms in 1812, for example, property. (One could buy a very substantial house (8 bedrooms plus) for £3,000 in 1812). Tea and coffee, on the other hand were very expensive. Postage was extremely expensive in real terms. In 1812 Britain was at war, and food prices were rather high. These comparisons are often tricky.