In article , says...

Foam in Allotment wheelbarrow tyres become flat proof ?

https://www.youtube.com/watch?v=bQbRfspyxW4

What kind of foam might we be able to get in the U.K. ?

My barrow used to be very susceptible to puncture from thorns, but
hasn't needed attention for a few years since using a liquid goo
preparation that is used by cyclists. I can't remember the brand name,
but application is simple - remove valve, introduce goo, replace valve
and spin wheel to evenly distribute goo. Re-inflate, cup of tea, then
ready to go.

Steve