19½ miles Folkestone Cap Griz-Nes. 52.3 miles St Catherine's point to the French coast, direct track MP.
FL55 is about your maximum if you're flying at proper levels on the first option. You could go to FL100 with the second option. Much higher.
Who knows what the wind will be doing on the day in question, but a conservative 100 knots in a PA28 means either 11½ mins or 31½ mins in nil wind. Halve that for the mid point on the long one and you're looking at say 16 mins from either coast. In reality, one is likely to be less because of the wind.
I think my POH for the PA28 said 500ft/min with no engine and windmilling prop, so FL100 might (might) give you a book 20 mins. It won't though, but it may be close to 16.
Aerodromes at both ends too.
But you won't be travelling at 100kts with a 500ft/min desent with no engine. Your forward speed will be much slower.
Also, you need a pretty good day to make FL100 if VFR only.