Unless your hours were cut temporarily (i.e. you have been on Short Time and your contractual hours haven't changed) then your Statutory Redundancy Payment will be based on your reduced hours, I'm afraid.
The rules about "a week's pay" are very complicated, I'm an employment lawyer but I have to check and recheck them myself quite often, but as a general rule if you have normal working hours and your pay does not vary with the amount of work done in those hours then you are entitled to be paid what you would be paid for the working week under the contract in force at the relevant time (i.e. now.)
If your pay does vary e.g. because you get commission, overtime etc then the calculation is based on the average of the previous 12 weeks although then again there are quite complicated rules about which weeks count,
If you get stuck you can always DM me and I can see whether I can help.