Wow...if those prices include VAT, the 25+ price only gives them about 10p profit. If that's their standard pricing it's hardly worth holding a TAG. Edit: It doesn't include VAT. Still only just 20% over wholesale though, that's very good.
Transfer more than 100 domains to them and there's a further discount They are very good. The only reason I got a tag is for dropcatching.
Have you tried the partners of this site? https://www.thenames.co.uk/ £4.55+VAT, which is cheaper than Yay, and they were one of the first to sign publicbenefit.uk
But renewal is £5.55+VAT while @yay it's £4.79+VAT. When I had a couple of hundreds of domains with them they discounted the price to £3.99+VAT. They are a young and relatively small registrar so they can be very flexible for big clients.