TNT can be done at less than £20 last time I did it early 2018if you book through parcel2go for a A4 size envelope weighing less than 1kg iirc. 2 days London to Bangkok, delivered at my condo. 32x32x2
Edit: I just did a quote and it seemed they've upped their prices from £20 to £29.40 in the last year since I used them last because they don't offer the option to drop it off at your post office now (courier will collect from your UK address) and maybe just a general price hike.
Sorry, but still cheaper than DHL and faster TNT express