Shielded from photographers gathered to record the transfer, which is receiving wide attention in Thailand, Gregory, 31, was shackled and taken in a police van from the notorious Klong Prem prison on the outskirts of northern Bangkok to an immigration detention centre.
Gregory, from Halifax in Yorkshire, was herself unable to comment on the transfer: "They've told me not to speak to you," she said, as Thai prison guards and British diplomats escorted her past waiting journalists.
"Sandra is very well and looking forward to going home. She will fly back later this evening," said one British embassy official yesterday.
Thai police arrested Gregory, along with fellow Briton Robert Lock, as they were preparing to board a flight to Tokyo in February 1993. Customs officials at Bangkok's Don Muang international airport found 102 grams of heroin inside baggage belonging to Gregory. She claimed to be carrying the drugs for Lock in exchange for medical expenses and her air fare.
Lock denied any involvement and, after three years of investigations and legal proceedings in Thailand, was found innocent. Gregory pleaded guilty and was sentenced to 25 years: the jail term later was reduced under a Thai royal amnesty to 21 years, 10 months and 20 days.
Three other Britons were also transferred to British custody yesterday and are believed to have joined Sandra Gregory on her flight home. Kevin Grant, Peter Heather and Andrew McGarrity are each serving between 20 and 40 years for drugs offences committed in Thailand.
Under an agreement signed in 1991, more than 30 British citizens serving prison terms in Thailand, mostly for drugs offences, can apply to serve the remainder of their sentences in Britain, provided they have been in Thai custody for more than four years.
Six Britons have already been returned under the scheme which has in the past been dogged by bureaucratic delays. However, the latest transfers have been welcomed by prison welfare groups in Britain who have praised the speedy handling of Sandra Gregory's case by the Thai authorities. Diplomats anticipate more transfers of British prisoners shortly.
"These are transfers, not releases, there will be no reduction in sentences," said Anthony Stokes, a British Embassy official in Bangkok.
"But all those transferred will qualify for normal British parole arrangements. That still means that in Sandra Gregory's case she may have to serve another nine years in Britain before she is free," he added.