I almost bought a similarly priced Argos ebike a couple of years ago, but enquiries about where to get spare batteries resulted in a high priced quote, so I bought a kit instead. If it has unusual fittings, they might be small run for what is after all, a captive market, therefore the prices compared to generic parts might be higher? Sorry, I'm rambling - I don't know the answer to your question, but I would check all documentation you have; instruction manual etc, then google the model number of your bike or otherwise find the manufacturer which Tesco bought your bike from to brand "Tesco" and sell on to you. If it's some Chinese company, they may have sold large numbers to a distributor in the UK, which Tesco may have bought them all from, but I doubt it because Tesco would most likely order at scale directly from a manufacturer to get the lowest unit price.
If you upload a few photos of what you are looking for, someone else might know - or suggest an alternative product which could fit?