Bedford to Luton is a tadge under 20 miles. If you assume 125 through Bedford and Luton stations no restrictions in between the maximum time saving is 3 m 24 secs. vice 100 mph max I got the impression the line was a lot slower than that on my admittedly rare excursions.
On the down (which is the more restrictive), it’s 110mph from Luton North to Leagrave Jn (about 2 1/2 miles), 120 from Leagrave to Sundon (2 miles), then 125 from there for the 14 miles to just short of Bedford station. These are the HST speeds.