would Lydd's current runway be long enough for E175/190 operating to AMS?
Probably do-able with a 175 off the current runway but the runway isn't necessarily the problem. Most of the time an approach to runway 03 is impossible for any commercial aircraft because there's an army firing range c.2km from the end of the runway, danger area up to 4000ft. So if the wind favours 03 they'd have to land on 21 with a tailwind. And 21 departures require an immediate right turn through at least 90 degrees to avoid the range. Not every airline's cup of tea.
Then there's the fact that this is an airfield in uncontrolled airspace with no radar. We've already seen Flybe withdraw from Dundee for that reason - and at one point Flybe was seen as one of the most promising potential operators from Lydd.