When we moved into our current house 6 years ago, I called BG to take out a service plan as the previous owners had a BG one upto the point of sale. They quickly aranged a direct debit and started taking money, and arranged a system check for a couple of months time.
When the guy turned up, he told my wife that the boiler did not meet current regs as it was sited in the kitchen (!!!!!!1111) and had to be replaced immediately. He shut it off and shut off the gas supply too, and left, leaving her in a cold house with 2 young kids in early winter. This given they’d been servicing it until a couple of months before!
I would not touch BG with a barge pole for ANYTHING.
Just had the boiler replaced now (it was 20 years old and all original parts until the fan went last winter so due an upgrade at last) for £2k by a local firm, including flush, magnetic filter etc – installed in the same space in the kitchen. 7 yr warranty as they are an approved Worcester Bosch dealer too, and very good service, everything clearly explained etc.
OP – get some more quotes quick!