From what I recall, this can go in three ways.
You put your house on the market as it is, price it slightly higher. Once you have a buyer, your buyers solicitor should pick up that it doesn't have building regulations.
You can then negotiate a more realistic price, they can go ahead with indemnity insurance and the sale goes through. Or the buyer pulls out.
Please note you cannot do this if you have previously alerted the council to the lack of building regulations.
A second way is you pay out in advance of a sale and bring the bathroom in line with today's building regulations. The difficulty with this is regulations change. As it wasn't ever built with any you may end up with a big bill. But it won't be more than the costs of an en-suite in the first place.
The third option is to get a builder to quietly put the room back as it was before the en suite was fitted. It becomes the room it was originally.
You won't get fined. You didn't know that it was put in without building regulations. We've had this with both buying and selling, It's a formality of paperwork, please don't worry.