You can count on the flight being at least 75% full. Easyjet are well established on the London-Amsterdam run, and they will practically give the seats away if need be to ensure a full plane.

Easyjet runs a similar sort of service to Southwest. Food and beverages on board must be paid for, and the seat pitch and width are industry standard (i.e. 31 inches pitch and 17-18 inches width in a 3-3 one class configuration). Armrests (except for bulkhead seats) can be raised and lowered.

You cannot allocate seating. When you check in your boarding pass will have a "Priority Number" listed on it, based on the order you check in. There is no on-line check in and check-in only opens between 2 and 3 hours before departure. At the gate, priority numbers 1-30 board first, followed by 31-60, and so on. At London airports this is strictly enforced. At many non-UK airports this is not so strictly enforced.

Exit row seats are not pre-allocated - the first people on the plane usually take them.

Actual flying time to AMS is around 45 minutes.