Ryan Lock, 20, from Chichester in West Sussex, died fighting alongside Kurdish forces near Raqqa on 21 December.
It is believed he turned a gun on himself to avoid being taken hostage.
Dozens of people, including members of the Kurdish community, held roses and framed pictures of him, as his coffin arrived at Heathrow Airport.
The former chef, who had no previous military experience, joined the Kurdish armed fighting forces, the YPG, after telling his family he was going on holiday to Turkey in August.
The YPG told the BBC that "trace of a gunshot wound was found under the chin", suggesting suicide.
Staff at Heathrow were rather caught off guard as Ryan Lock's body arrived home.
No-one at the cargo bay of Royal Jordanian Airlines expected to see a crowd of mourners arriving on their forecourt.
With just a couple of hours notice, Kurds young and old turned out to pay tribute to a man from Chichester whom they had never met.
"This is not just a foreign war, this is a fight for humanity", one told me.
"International solidarity" they chant as they place flowers over the private ambulance carrying Ryan Lock's coffin.
They hold their hands up in silent salute.
To have a British man fight their cause, touches their hearts.
A Kurdish mother kisses the ambulance and weeps as it drives away.
Supporters of the YPJ female-fighting force paid tribute to Mr Lock, saying his memory would "forever live on in our struggle for the freedom of Syria and our hope for change in the whole world".
YPG general command member Mihyedin Xirki had previously said Mr Lock was a "martyr" who died "putting up a brave fight".
The volunteer, who attended school in Havant, Hampshire, became the third British man to die fighting alongside the Kurds against so-called Islamic State.
The Foreign Office continues to advise against all travel to Syria.
The body of a Briton who died after travelling to Syria to fight so called Islamic State has arrived back in the UK to a hero's welcome.