It's definitely not too late for sowing seeds. Many shouldn't be sown until April at the earliest.
Try Lidl and Wilko for seeds. Also Poundstretcher has loads usually. Also fruit trees etc.
My favourite seller on eBay is sadly closed due to CV.
It's worth an eBay search to see if anybody is still trading.
It looks like Sutton's (www.suttons.co.uk) is back online tomorrow.
Marshall's (www.marshallsgarden.co.uk) is trading still.
There are options. Though the cheaper ones aren't about as much now.