There is a big difference between digital currencies "CBDCs" and cryptocurrencies, and not all CBDCs should be using blockchain, and even if the UK thinks so, the solutions will be centralized one.
The closest blockchain protocol to centralized solutions is Ripple, but this does not mean that UK will rely on any of the existing models in the form of cryptocurrencies to be the basis for building their digital currency.
But what blockchains they use ? Could it be ETH ? Maybe tron ?
It is impossible for this to happen.
Examples of some digital currency projects that are not based on the blockchain
https://www.indiabudget.gov.in/doc/bh1.pdfhttps://www.bankofcanada.ca/2021/12/staff-working-paper-2021-65/https://www.bankofengland.co.uk/research/digital-currencieshttps://www.federalreserve.gov/publications/files/money-and-payments-20220120.pdf