Metv through my thai airways flight london to phuket via bangkok can someone shine light on process transiting through ..kind regards in advance immigration and any good advice

Hi there , I’m currently in Bangkok, planning to fly to Cambodia in a few days for around 2 weeks then I need to be back in Suvarnabhumi for a flight out of the country to Brazil. Anyone know how the transit works? I have a 30kg suitcase with me so not sure if I’d need to check out then back in again as I’m getting conflicting advice on google. I’m British by the way. Thanks in advance.