Not setting records, but there are frameworks in other countries for completion time based challenges within the brevet time limits. I came across the American Cyclos Montagnards brevet challenge recently. Whilst it may only appeal to a niche subsection it seems some other countries have adopted similar approaches.
'The R80 honor requires completion of a brevet series (200, 300, 400 and 600 km) with each brevet completed in 80% or less of the maximum allowed time limit. The R70 and R60 honors are for those who complete the brevets in 70% or 60% of the allowed time, respectively.' More here;
http://cyclosmontagnards.org/R80Rules.htmlRough average;
R80 19km/h total average
R70 21.5km/h total average
R60 25km/h total average
Whilst there is no validation structure in the UK it could be an interesting personal challenge if that way inclined.
In these times of reduced stopping options it could be an interesting exercise in terms of honing self sufficiency skills, reducing stopped time or covering routes with a slightly different focus.