The solicitor you used to conveyance your new house, mortgage adviser or estate agent. If you go to church, your minister. Anyone from a past place of work that had some kind of responsibility. Your old accountant. If you have any social activity you engage in (gym, chess club, whatever) someone there of any kind of standing.. Even a librarian, butcher, baker, candlestick maker etc can sign; it's a pretty low bar, I can't believe you literally cannot think of a single person who knows you, and also who has a sliver of credibility
The govt advice also says you don't have to have your photo signed if you have authorised the DVLA to contact the passport office to verify your identity