Whatever train is used for the route from Bedford has to be not-electric. Hence, at best, something like a 158/9 (90mph max) and that is the comparison vs. 230 needed.
Some "back of a fag packet" quick calculations* suggest the difference in elapsed time would be of the order of 12 seconds. Such a differential could equally easily arise in - for example - the length of time it takes one crew vs. another, or for one train length vs. another, in changing ends, provision of wheelchair access, signalling delays and so on. Even supposing relaxed timing for the reversal accounts for all of that, it still leaves a maximum differential of, say, 12 seconds which is, I'd say, negligible.
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* Assume 0-60 and 60-0 are about equal for 158/9 and for 230, and that in total these account for two of the three miles, give or take, based on that 98 second 0-60 time quoted elsewhere.
That leaves 1 mile in which the 158/9 continues to accelerate (let's say up to to 90) and then slows (earlier) whilst the 230 proceeds at 60 throughout. Approximate average speed for the 158/9 up to 90 and back is say 75mph vs 60 for the 230. 1 mile at 75 takes 12 seconds longer than 1 mile at 60.
