The post office has many rates I can use for mail from Chiang Mai to the UK, eg for a 250gm package you can:

1. Letter air mail B185 - can be sealed and contain a letter
   
2. Small package air B136 - can not be sealed, string or staples only, no letter and must have customs form
   
3. SAL (letter, small package or printed paper) B97 - goes surface to Bangkok, air to London and surface to Manchester, just the same as normally happens.
   

I used to use letter rate but have now moved to small package rate. I've found that the delivery time is exactly the same as a letter.

I'm now considering the SAL rate - has anyone tried the SAL rates? Is the delivery time similar to a letter?

They have SAL letter, small package and printed paper and all SAL rates are the same, a good half the letter rate. If I use SAL letter can I include a letter and seal the package????

I'm very happy sending packages to the USA via SAL, usually 1 kilo or less. They are said to take 30 days to arrive and 90% of the time do just that. I'd say the rare 1% arrive late for some reason or other. The other 9% arrive a week or so early. You can do the whole tracking n insurance thing.... had to trace a lost package once, and found it a highly INeffective system that only allowed me to see the date the package left Thailand, not any other info about where it was/went/ or when. Luckily the cosmos got it back on track 3 weeks late !

For actual letters or documents I use the letter mail rates/service.

Ah, SAL seems slow. Normal and small package to the UK takes 5 - 7 days. 30 days for SAL is probably not acceptable.