That has got some seriously slack angles
Slack, yes, but not out of the ordinary for a bike of it's era. For comparison, here's a Raleigh Sabre dating from 1994, with a welded 18-23 Hi-tensile frame. The angles are 70 head/70 seat, which appears to be the standard geometry for most, if not all the welded 18-23 Raleigh MTB's from the 80's and 90's.
The Jack Taylor does have unusually long chainstays though, much like a Dawes Ranger which has a similarly long wheelbase.
I find the 70/70 geometry Raleigh frames to be a very good ride on both gravel and potholed tarmac. Nothing lively about them, but the design takes the sting out of poor roads, much like a slack 3-speed roadster frame does..