I'd also be interested to know if any militia lists have survived for Middlesex.
These are not lists of people who actually ever joined the Military.
Just in case anyone isn't aware of exactly what the Militia Lists are, these quotes may explain these lists:
"Under the 1757 Act the parish constables were ordered annually to record the names of all men aged 18 to 50, excluding only those who were peers, clergy, teachers, apprentices and 'peace officers'. However in the 1758 Act (31 Geo II c26) and thereafter until 1831, Parliament directed that no names be excluded, although the upper age limit was lowered by 5 from 1762.
The militia ballot lists should therefore, in theory, be a complete annual censuses of all men aged 18 to 50 from 1758 to 1762, and aged 18 to 45 from 1762 to 1831."
Quote from 'Militia Lists and Musters 1757-1876' by Jeremy Gibson and Mervyn Medlycott, published by the FFHS
The Militia Lists were held and recorded in each and every town through England. If your ancestor was recorded on such a list it does not mean that they served in the military forces, just that they were a resident of that town and were available to serve should an invasion (by the French) take place.
Certainly not every Militia List has survived to this day. The ones that are still available have usually been transcribed and would be held at the local Family History Centre or Archives Office within each town or city.