Colonel John Tiltman wears a very strange uniform in this picture, dated 1919.
The original webpage where I saw the picture is http://www.colossus-computer.com/colossus1.html.
The Colonel was of Scottish descent and served with the King's Own Scottish Borderers in WW I (according to Wikipedia). The trouser pattern in question could well exhibit the unit's (mainly green-and-blue) tartan. Also, the cape he wears appears very similar to those exhibited at the King's Own Scottish Borderers Regimental Museum's web site. And as for him wearing trousers instead of a Scottish kilt I'd say: maybe it's some (Scottish) Winter uniform :)
P.S. Nice web site about the Colossus computer, BTW.
Not all Scots ran around in kilts - that is very much a Highland tradition. The KOSB being borderers and lowlanders in general did not see the kilt as part of their own tradition, and thus Tartan Trews were worn - and looked very sharp if I may say so. The pipers of the regiment did wear the Royal Stewart in a Kilt, bit other ranks wore Leslie Tartan Trews (in honor of the Earl Leslie that formed the original 25 regiment of foot in 1689).
Sadly the KOSB were both downsized an amalgamated into the new Royal Regiment of Scotland, and most traditions and certainly the uniform were lost to time. You can still vist the KOSB museum in Berwick on Tweed.
Source: a 5th generation borderer!
The King's Own Scottish Borderers (formed in Edinburgh in 1689) were a Lowland Scots regiment one of the original truly Scottish regiments formed before the union of Scotland with England (unlike the much younger and junior Highland regiments who were never part of the Scottish establishment) and as such wore trews instead of kilts.
The original Scottish Regiments: Hepburns Regiment which became 1st of Foot (later The Royal Scots (The Royal Regiment), The Earl of Mar's Regiment which became the 21st of Foot (later The Royal Scots Fusiliers), Leven's or The Edinburgh Regiment which became the 25th of Foot (later The King's Own Scottish Borderers), and The Cameronian Regiment which became the 26th of Foot (later The Cameronians (Scottish Rifles)). Were all formed before the Union of Scotland and England all fought against The Stuart cause, they held the predominantly Jacobite and Catholic Highlanders and Catholicism in contempt regarding the Highlanders mode of dress as barbaric and uncivilized. The original Scottish Regiments were dressed very similar to their English contemporaries and carried on that tradition until the 1880s. They did, however have Pipers from their earliest days and always used Scottish drum beats.