I don't care that I'll be swapping 150/150 XGS-PON for a less capable asymmetric GPON service that will probably cost more. If the new provider's CS will actually talk sense and will fix problems efficiently should anything go wrong, it will be worth it.
Check out
Aquiss. IMO the sweet spot is £45 for 300/50 (you only save £3 by halving speed to 150/30). Great customer service, dedicated static IP4 and an IPv6 /56 subnet, very reliable network.
It's a 12 month initial contract and no automatic price rises within contract or at end of contract - just roll on month by month. Currently they are offering first 6 months for half price. They have been known to drop it to 3 months or remove the offer completely; if they do, I'd expect them to reinstate it for Black Friday.
Caveats:
* You have to provide your own router
* There is a £45 cease charge if you disconnect (without migrating to another provider) or move house - regardless of whether they can serve you at the new property or not.
A newer but apparently well regarded option is Olilo. They have two types of service, "Consumer" and "Prosumer" running on two different networks, which is rather confusing. "Consumer" is significantly cheaper than Aquiss, although there are £1/year price rises, and it's £2 extra for static IP. "Prosumer" gives you 900/100 speed for about the same price as 300/50 on Aquiss - but you said you don't care about the speed anyway.
Again, you provide your own router. There's no initial 6 month discount, but there's also no cease charge (unless you take the 1-month contract option and terminate within the first 12 months).