Epower – you should have included a "rabbit hole alert" before posting this one. Unable to find an answer through my normal sources, I did a search on Hansard on the UK Government site. On 10 July 1918, a Mr Peto, MP, asked the First Lord of the Admiralty "whether he is aware of the fact that at the end of the War Territorial and New Army officers are entitled to a gratuity equivalent to 124 days' pay for the first year of service and 62 days' pay for each subsequent year's service." A scale for senior NCOs and junior ranks appears in the Hansard entries for (if I recall correctly) 20 November 1918.
There is a war gratuity calculator available for download at
war gratuity calculator. It seems that you can obtain a trial membership at this site.
The bottom line is that the gratuity would be based on length of service and pay scale (i.e. rank). Lewis, for example, had more than two years of service as I recall, having been in France long enough to learn to fly a Morane throughout the 1916 Battle of the Somme. Add to that the time he spent in training before shipping to France and the time he spent in England before demobilisation. So £600 would represent the war gratuity for a captain with about three years of service.
Cheers,
Raine