Either a full medical disclosure and certain insurers will accept things while most will permanently exclude
Or a moratorium - however it's normally (uk anyway) anything last 5 years excluded for first 2 years if you can go 2 years without symptoms treatment advice or medication
If its a chronic condition you've got its never going to he covered unless you can get medical history disregarded (large cooperate scheme ridiculously expensive)
Source: uk insurance broker for 7 years and recently looked into international PMI for myself
can confirm in june applied for tourist evisa in uk, couldn't wait for it so had to have an outbound flight booked ,in august extended the 30 day exempt, in september did a border run using the tourist visa on the way back, have just done a 30 day ext on that
which country you from Lee? if you have the ability to apply for a tourist e-visa do that and when it comes through and you're in the country just do a border run when you want to start it
To add what Todd said one of the hurdles you're going to have is getting a bank account in thailand that quickly - most wont let you get it on a 30 day exempt but some will on a tourist. (Bangkok bank) is very friendly however you'd need a certificate of residency and to buy their insurance depending where you go before you can open up a bank acc