Presuming an international segment, OP is entitled to the fair market value of the suitcase and its contents up to the Montreal Convention maximum (approx. GBP 1131).

Fair market value is what a reasonable person would pay for a given item. Thus, a shirt purchased for GBP 100 a year ago and worn 20 times, is hardly worth that today.

Either OP will need to document the value of the items in question or accept some sort of average. Alternatively, OP would have to file a court action and prove the value of each item. Not likely to do better.

As others note, there are lost luggage insurance policies which provide for 'replacement cost". But, to be frank, most policies also only cover fair market value.