I've come off two motorbikes in the UK. On the first occasion I wore a leather jacket, on the second I had a synthetic jacket on (because it was more comfortable in hot weather).

Both were completely safe. On both occasions I slid along the tarmac for about 10-15 meters, I was travelling at around 30-40 mph. I still wear the same leather jacket 30 years later (not for riding) but the synthetic jacket was a right-off.

On both occasions I really smacked my head: don't mess about with sub-standard crash helmets.

So even though leather is better, we're not racing the TT, we're just going from A-B and if you want to wear synthetic you'll be fine at normal speeds. So if you can't wear leather, for whatever reason, don't let that stop you.