The machine will work fine with either polarity, but as mentioned there might be a preferred way round for the mains lead to put an SP power switch in the line. DP switches weren't unknown. The USA had a convention for terminal screws identified by metal colour. Copper = line; nickel plated = neutral, but I doubt that will apply to a UK tape recorder mains lead.
Many tape recorders were class 0 but I've never met a live chassis (transformerless AC/DC) model. Radios and TVs yes, but the need to interface tape recorders to the outside world at AF rather than RF, and their generally post-dating DC supplies, and generally having induction motors, meant there was no point not having a mains transformer.
Most importantly, if the machine has not been used for a long time DO NOT PLUG IT IN!
Old valve electronics that have stood for decades, almost certainly will have faulty or unformed capacitors due to both age and lack of use. If powered-up without these first being re-formed or replaced as appropriate, knock-on damage often occurs that is far more expensive and time-consuming to fix and sometimes writes the machine off. E.g. a leaky audio coupling capacitor that needs changing could cost £0.50 from RS, but if it toasts the output transformer (even while the unit appears to be working) you have a whole new project to find a suitable transformer, which cannot be bought off the shelf and might be fairly specific to that model, plus possibly a £15 valve.
There is a set routine for preparing and waking up long-dormant valve electronics. TBH I don't know the last time I found a deck of that age that would have worked as-found, unless it has been in the hands of a collector who has kept it in use. I like old tape decks, I have over 100 machines from tiny domestic ones up to studio-quality, but (mainly on account of belts, heads and pinch rollers) they can be expensive and hard work to restore.
What model is the machine, and what is the objective of getting it running?
