This has happened to me recently.

Nothing to worry about - it is identical to the J fare and will print Business Uk on the BP.

I think it is to do with our travel agent. When they send our company a statement which shows J class as "business" our accountants get grumpy because we aren't supposed to travel business in the UK and no matter how many times they are told that UK Dom J fares are not business class - every month they moan. So the TA is booking these in Y class now to avoid arguments.

Y class has always been available on UK Dom and has always been Biz UK but usually only when bought on a through fare to another destination.