B&Q Spare Parts?

Amazon might have something https://www.amazon.co.uk/s?k=bbq+rack. You could get an oversized rack and cut it down. Local tip or electrical retailer might have an old stove they've take to recycle that has oven trays you can cut to size - if you can persuade them to let you have a look?

I doubt it.

BnQ are just a stack em high and sell em not so cheap retailer. They have no interest in fiddling around with parts of the things they sell.

To be fair, it could have been a BBQ from B&Q