The UK personnel were embedded with the forces of allied nations, including the USA and Canada, which have been conducting strikes against the Islamic State (IS) terror group - also known as Isil or Isis.
However MPs voted against military action in Syria in 2013, and parliamentary authorisation has so far been given only to UK air strikes against IS in neighbouring Iraq.
But the Ministry of Defence said soldiers embedded in foreign nations' forces were effectively operating as troops of that country.
The prime minister David Cameron and Defence Secretary Michael Fallon have indicated they would call another vote on air strikes in Syria but that's unlikely to happen until at least the Autumn.
Tim Farron, a churchgoing Christian, said: "First of all, given the parliament has not given approval for engagement in Syrian air space at all, that would be a breach of trust with the British people.
"But secondly, whether this is the right strategy, even if there had been approval."
Asked whether he would be guided by his religious convictions when making decisions on whether to commit troops to military action, Mr Farron told BBC Radio 4's Today programme that he had heard reports of UK pilots being involved in airstrikes in Syria, adding: "The decisions you make are not based on some kind of overall theological sense of what you should do.
"It is 'Is it right for us to attack a sovereign country? Is it right for us to overstep a mandate that we have not been given by the electorate or by Parliament? Is it right to incite Isis - or not to incite them, but to play into their hands and make them to be martyrs?'
"You make these decisions on the basis of what is right and the basis of what the evidence says in front of you."
Details of British personnel's involvement in strikes by allied nations' forces were revealed by a Freedom of Information request from pressure group Reprieve.
A Ministry of Defence spokesman said: "When embedded, UK personnel are effectively operating as foreign troops.
"The UK is contributing to the anti-Isil coalition air campaign against Isil targets in Syria through the provision of intelligence, surveillance and reconnaissance.
"Isil poses a direct threat to the UK and to countries around the world. The UK is not conducting air strikes in Syria.
"But we have a long-standing embed programme with allies, where small numbers of UK personnel act under the command of host nations.
"That has been the case in Syria, although there are currently no pilots operating in this region.
"When embedded, UK personnel are effectively operating as foreign troops."