Savers are protected under the Financial Servies Compensation Scheme (FSCS) to a current limit of £85,000. The compensation limit applies to each saver for the total of their savings with a building society or other mutual deposit taker, regardless of how many accounts they hold or whether they are a single or joint account holder. In the case of a joint account FSCS will assume that the money in that account is split equally between account holders, unless evidence shows otherwise. This means that each account holder in a joint account would be eligible for compensation up to the maximum limit. The maximum amount covered for a couple, for example, would be £170,000.
Protection under the Compensation Scheme is per authorised deposit taker under their deposit taking banking licence, not by brand name. Therefore savers need to be aware of which trading names and companies are within each licence if they wish to ensure that they remain within the limit set by the scheme.
No ordinary investor in a building society savings account has lost money since at least 1945, and probably for a long time before that.
Moneyfacts publish a "Who owns whom" list on their website. This details which company has the licence and which other brands and companies operate under the same licence - https://moneyfacts.co.uk/savings-accounts/guides/who-owns-whom/
The FSCS have a deposit protection checker to enable consumers to check whether their cash savings are fully covered by the scheme - https://www.fscs.org.uk/check/check-your-money-is-protected/
People with some types of temporary high balances will have FSCS protection up to £1m for up to six months. Things like the proceeds from a house sale qualify for the new protection limit.
Further information can be found on the Financial Services Compensation Scheme website linked to below.