Pay-as-you-go plans are generally available for mobile phones - but not for “data only” mobile broadband. As such, plans of this type don’t appear to be easy to find within the UK. 3 offers what appears to be a preloaded SIM that will require topping-up every 30-days.
https://3g.co.uk/data-only-sim/pay-as-you-go
Other Cellular Operators typically offer DataSIMs on a rolling plan - but none, to my knowledge, offer a non-expiring preloaded DataSIM that can be topped-up on demand. EE appear to offer time-limited pre-loaded DataSIMs - such as this available via Argos:
https://www.argos.co.uk/product/2152310
Otherwise, there are short-term and/or fixed-data plans that can be enabled directly from iPad settings (using the iPad’s eSIM) from MVNO Operators GigSky, ReadteaGO and Truefone.