You need to establish the spindle size that goes into the knob or lever and also if the center is bigger.
Standard door handle spindle size.
The standard size you need to look out for is 57mm and this is the size between the lever centre or the spindle and the lock centre and thankfully that is a very popular uk size.
The standard thickness of a spindle is 7 6mm most uk latch manufacturers produce latch openings at 8mm to accommodate this.
Along with a large selection of door handle spindle expansion sleeves adaptors which allow a smaller spindle to be used with standard door furniture.
The majority of spindles are 9 32 square.
Door handle spindle reducers are available in sizes 5mm 8mm 6mm 7mm 7mm 8mm 8 9mm and 8mm 10mm.
Spindle 3 1 2 inch straight spindle set with screws for old style mortise door knobs view item 42940 rosette for door knob 2 1 4 inch dia.
The center of a swivel spindle the part that ends up in the lock hubs may be fatter then the spindle ends.
Spindles and followers were not always standardized to 8mm.
However we know that there are a number of latches particularly from the early 20th century that had a slightly smaller opening meaning that a 7mm spindle is required.
A smaller portion are 5 16 square mostly for levers.
Lever to lock size is 57mm on a typical interior locking door handle.
It should be noted however.