You entered into an assignment agreement with the client. If you did not cancel your shift before the cancellation term, it’s your own responsibility to find a suitable substitute. The client is expecting someone that is on the same level as you. If you were unable to find a replacement, this will have 2 consequences:
It will have a negative effect on your attendance percentage.
The client can request a fee of £50 as compensation. This mostly depends on if you informed the client beforehand or not.
Pay attention: If you cancel a shift before the cancellation term goes into effect, you won’t be expected to arrange a substitute.