Doorbells don't typically have the transformer built in, it is a separate item usually near the main electrical panel, inside the doorbell's chime box, the garage, or the attic. For the 2nd gen wired Nest Doorbell, you need a transformer that is rated for 16-24 V AC, 10-40 VA to provide enough power.
I think part of the reason a transformer that meets those requirements is more difficult in the UK is because the wired 2nd gen model has higher power requirements than most doorbells. It's also not currently officially sold or supported outside of the US and Canada.
Be aware that since the product sold in the US and Canada doesn’t have UKCA/CE approval, in the unlikely event there’s an electrical fire in your home which might plausibly be related, your insurer may deny any claim, or at the very least is arguably likely to put the burden of proof on you to show the doorbell wasn’t responsible.
