Both the Tibetan stamps and the Japanese Occupation of Burma have been very extensively forged. The Nepal and Tibet Study Circle Web site
http://fuchs-online.com/ntpsc/ may help to determine if this is genuine. I'm not sure about online resources for the Peacock overprints. That appears to be an SG J25, which would be a £225 job if genuine. Obviously one to get a certificate for, from the RPSL or BPA only.