Hi
You get what you pay for really.
As FTTP is pretty stable and you get the line rate speed you buy, the support the ISP gives is probably less important, as you are less likely to need it. But you can still get congestion issues and slow downs if the ISP doesn't have enough capacity.
Do you need a static IP and IPv6? Is it business or pleasure?
Personally I've not used I mainstream ISP for many years and it is has been worth the extra cost to just have people answer tickets in 5 minutes rather than 5 days, it is night and day. It's like most goods and services, you only get to find out how good they are when you have a problem.
If want the cheapest and don't need a extras like static IPs, then like buying Tesco's basic baked beans or Sainsbury's basic baked beans, they are not going to be very different. So you might wish to see what offers they have on and might just come down to who offers the better router?
Regards
Phil