That's plenty of money to work with and there are plenty of good passive isolating headphones to choose from that seal well enough to use on public transport and sound great. Here are a few choices I think would really serve you well along with some short notes about them:
Soundmagic HP100 - These headphones are kind of virtuosos in performance. They sport a really robust sub bass response when called for and sound really good with both classical and jazz as well. Their soundstage is spacious and quite wide for a closed headphone, even though their instrument separation isn't all too great. Like the Lindy headphones their pads are also quite thick and will get hot over time, but they isolate well and feel really soft and lush. I think their biggest caveat is that they don't sport a very high sensitivity and definitely improve a bit when amped.
£149.99
AKG K550 - This is a fantastic sounding and fairly well built headphone that offers a mostly balanced sound with a very respectful soundstage for a closed headphone. They're quite comfortable and isolate well if you can get them to seal properly around your ears. That's the only caveat with these is that their lack of a concave shape makes for a tougher seal for some head shapes.
£99.00
Lindy HF-100 - These are a fantastic value rebrand of the famed Brainwavz HM5. This headphone also features a mostly balanced sound with a slightly warm sound signature and decent soundstage. They isolate remarkably well and are fairly comfortable. The only caveat about them is their thick pads might get really hot after a long listening session.
£74.90
These would all suit you well:
VSD3S
Astrotec AM-800
T-PEOS Popular
ZERO Audio Carbo Basso
Xiaomi Pistion 2
