B4RN do not give a price guarentee, they just say "look at our track record over the past 9 years".
The B4RN gigabit broadband service costs just £30 per month
B4RN is a non-profit community benefit society. When they started connecting people to their network in 2012 the cost of their broadband service was £30 per month. It is still £30 per month today and will be £30 per month for the foreseeable future. All other ISPs increase their prices every year.
BT's full-fibre broadband is much more expensive
BT is providing a set of packages over the Openreach network where Openreach has installed full-fibre connections that they call Fibre First:
BT, like all the other for-profit providers increase their prices every year.