Our 1930s house has odd-sized internal doors that don't match the shops, can a joiner fit standard ones?

Yes, a joiner can nearly always hang standard off-the-shelf doors in a house of that age, and it's usually cheaper than having them made to measure. The thing people get wrong is measuring the old door rather than the opening, because a door that's been on for decades may itself have been trimmed down or dropped on its hinges.
First thing I'd check is each lining for square and its size off the floor, because openings in old houses often aren't true. Most engineered internal doors have a trim allowance on the lippings, usually around 3 to 6mm an edge, so a good joiner buys the nearest size up and planes it to fit. Push past that on a hollow-core door and you break into the hollow middle, so if a gap's big you want a solid or semi-solid door, or a new lining fitted.
Don't assume one measurement does the lot, though. Doors wander in size room to room in older houses, so measure every opening separately.
On labour you're looking at roughly £80 to £200 a door to hang and adjust a standard one outside London, more if tired frames need swapping too, and a joiner will usually get through three to five straightforward doors in a day. Ask whoever quotes whether the price covers trimming, new hinges and a latch, and taking the old doors away, because that's where two quotes that look the same quietly differ.
If you want a couple of prices to compare, we can match you with vetted joiners who fit internal doors in older homes — see our internal door fitting cost guide to get quotes.