I don't know about the bottle warmer, but have you tried using the little disc in the avent cap while travelling, not packing it away with the teat attached?
Because avent bottles shouldn't leak at all. My kid is 12, and I still use an avent bottle (with the little disc, not a teat!) for carrying lactofree milk around for my coffee when I'm out. Hasn't leaked once, and it's well over a decade old.
(I'm presuming here that the discs are still being sold...)
moomin35 Your baby is only about 10 weeks old, yes? (Seen other threads you've been on.)
It's really not safe to be trying to carry a bottle of made-up formula around, especially not if you're also trying to keep it warm, for such a tiny baby. Either carry a flask of boiling (or near-boiling) water and carry powder, and a sterilised bottle separately so you can make up feeds as you need them, or carry a carton of pre-mix when you're out and about and warm it up somewhere they have facilities to do so.
you can get insulated bottle carriers but they should be either used to keep WATER HOT and ready to mix with the powder when required or to keep MILK COLD until it is time to warm for a feed. You should not be trying to keep milk warm as that is an ideal breeding ground for bacteria. You could also do as a pp suggested and take an empty sterilised bottle and a carton of pre-made formula out with you