Phil Miller has an interesting find on UK-Guatemala cooperation during the Cold War for Vice News. Like Ronald Reagan, Margaret Thatcher would seem to have an interest in working with Efrain Rios Montt to contain the spread of communism in Central America, especially with regards to Guatemala.

Belize gained its full independence from the UK in September 1983. During and after independence, Belize hosted many British soldiers. Approximately 1,500 British soldiers were stationed along the Belize-Guatemala border and their main job was to protect Belize from a Guatemalan invasion. The two countries had and maintain a territorial dispute.

The troops were also off on counter-narcotics missions. Perhaps unbeknownst to some of the British soldiers, however, they were also defending Belize against incursions by members of the Rebel Armed Forces (FAR), one of four Guatemalan guerrilla groups. Thatcher and the British / Belizean governments feared that the presence of FAR guerrillas in Belizean territory would provide justification for the Guatemalan government to launch an invasion of Belize.

This is the same time period when the Sandinistas thought that if they pursued Contra forces across the border into their Honduran safe zone, it would have provided a justification for Reagan to invade Nicaragua. Feeding the Sandinistas' fears, Reagan invaded Grenada in October 1983. We are also talking one year after the Malvinas War between the UK and Argentina. Things have really calmed down, no?

Anyway, Thatcher and the British government used the opportunity to reach out to their Guatemalan rivals in search of areas of cooperation that might deescalate tensions.

Left-wing Guatemalan rebels were trying to topple Montt's regime and allegedly staged some of their attacks from Belize. The files show that British commanders feared these cross-border raids would give the Guatemalan leadership an excuse to invade Belize. To prevent this, top British army officers decided to share intelligence on rebels with Guatemalan commanders, even though they were linked to human rights abuses. Royal Air Force pilots made reconnaissance flights over the Belizean jungle looking for guerrilla camps and British troops carried out secret foot patrols. UK soldiers even used a Guatemalan rebel informant for one patrol, before sending him back to Guatemala where he was arrested and later murdered by government gunmen.

...

The files also reveal that a British policeman conducted urban surveillance of guerrilla sympathisers in Belize under "Operation Octopus". And even though diplomatic relations with Guatemala had been cut, the foreign secretary allowed British soldiers to play Christmas volleyball matches with enemy troops.

I can't say that I remember hearing anything about Guatemalan guerrilla operations in Belize. Most of their foreign operations seem to have been in Mexico, where the government tolerated (and at some points) encouraged their operations.

Gus Hales, a "2 para" veteran who served in Belize in 1983, now suspects his jungle patrols were unwittingly aimed at guerrillas. "We were told to look out for drug smugglers who may well be wearing uniforms. But the guys we came across in jungle camps were ordered and tried to conceal their tracks," Hales recalled

"They were Mayan Indians who knew how to live in that terrain, which made it kind of strange. Now it would make sense that they were guerrillas."

The files and interviews cited in the article don't give the indication that there was too much guerrilla involvement in Belize. It's possible. The FAR/URNG were on the run in late 1983 following five years of increased government violence, first in the cities and then in the countryside. Using the safety of Belize seems reasonable.