Each year the RYA publish the new Portsmouth Yardstick (PY) numbers around this time (March). The new table has arrived and you can see it here. Boat Timer keeps a table of these rating numbers and also has an easy way for you to change the numbers for your club (each club can adjust the ratings to suit their racing) and add boats not on the table. You can even add a new rating system.
The Portsmouth Yardstick is not the only system, other countries such as the USA have their own, though many are based on PY numbers. Another popular system in the UK is “Great Lakes” which is a variation of the PY based on the Great Lakes winter series racing and Boat Timer includes the Great Lakes numbers for many classes. You can see more details about it here.