I'm especially interested in the first half of the fourteenth century.

It's possible to estimate it, but not very reliably, assuming that you are interested in major battles, rather than small-scale private violence which was much less well reported. The numbers we have for major battles aren't very reliable either, but the size of English forces in major battles can be approximated as:

• Versus Scotland: Halidon Hill and the Siege of Berwick, perhaps 10,000 each, and probably less than 10,000 English forces in other battles.

• Versus France: The largest English commitment was to the Siege of Calais (and many of the veterans of Crecy were probably there), with a peak of perhaps 30,000 Englishmen in the land forces, and another 20,000 in the naval forces. To this we could add the major naval battles of Sluys and Winchelsea, with perhaps 25,000 Englishmen involved.

We have two major difficulties. We have no information about how many soldiers fought in more than one of these battles. Perhaps worse, we have very little information about deaths due to disease and starvation. If we take the 50,000 English at Calais, and add another 20,000 for the Scottish battles, we have a reasonable estimate, but perhaps high, not accounting for disease deaths (or simply not returning to England from France). if we're happy with a + or - 50% estimate, this seems OK. With a population of England of about 5 million in the early 14th century (not certain, either, but probably more reliable than the estimate of the number of veterans), this means that maybe about 1.4% of people in England were veterans with experience in siege or open battle. If we exclude women and children, perhaps about 5% of Englishmen were veterans.

Many thanks for this, and glad I asked--this is much lower than I estimated (must be a medievalesque penchant for grossly overestimating numbers). And thank you for getting past the careless headline--obviously I omitted the period, but couldn't edit.