Its not difficult or time consuming, no need to take a thai person. I arrived early and was all done in about 1 hour, it took longer because I had to go get cash as they don't accept card. They'll ask for an address in Thailand to send the certificate to, I paid extra and they sent it via DHL to UK but appeared reluctant to do initially, not a normal service. Arrived about 3weeks later in UK. All very helpful, no problems.
Mae Sai crossing is closed for tourists, Chiang Khong on Laos border is probably easiest / closest assuming you're in CM or CR. Rent a car and drive to the border, or go by minivan with Chiang Mai Visa Run, or other.
I've sent them an email but I really need to be sure, I'll be leaving it as late as possible before applying as unsure how it will be considered in UK, if gap before leaving Thailand / or if there's any expiry. HR in UK can be quite fickle about PNC checks.
I've seen that but appears to be for those planning to continue to reside in Thailand, work / education etc. There's also a link for those residing abroad with less arduous requirements which would be preferable but can this be submitted whilst still in Thailand and then forwarded to UK address or is there any other option?