Aye imac_boy,
Having re-read your posting, I am confused!

The letter from St.James's Palace refers to the award of the OBE and yet the envelope addresses your Uncle as having been awarded the MBE!

I leave that mystery to you.
After a bit more digging, I believe I have come across the entry in the Supplement to the London Gazette, 12th February, 1919, page 2231 that I believe refers to your Uncle. I have attached a pdf copy of the page, with a link in a 'sticky note' to the London Gazette website. Might I suggest that you widen the date range that you use to search the London Gazette by? I am sure there are other references in there.
I hope this helps.
My regards, Bill