I don't recognise the "safe days" of UK books.
In the past 18 months. Netbet, Luvbet, Lion have all gone down with all customers losing everything - luckily I was not with any of them.
but I was with sports.com who survived only due to UK betting buying them out.
I am with the following UK books:
Hills, Corals, Ladbrokes, Sporting Odds, Stan james, Chandlers.
I have concerns about Sporting Odds due to being part of the shaky sportingbet empire. Other than that IMO the above books are all solid.