I believe with Orange you can upgrade your phone once you're 6 months from the end of your contract or something like that. Orange isn't getting it til some point next month though, T-Mobile are getting it earlier for some reason. You should be able to check on their website, though the Desire isn't showing up on there just yet.
You'd be able to get it on that plan but you'd probably have to pay somewhere around £100 to get it if you don't upgrade your contract. I got the Hero for free on a £30/35 (I forget) contract and that was on a special offer.
It's worth calling Orange and asking about it, as you're close to the end of your contract you can probably get a good deal out of them. Failing that, go into Phones4U/Carphone Warehouse, they offer some really good deals on contracts and such, I've had to jump networks twice in the past 2 years before my contract was up, both times they paid off the last few months of my contract and gave me a free phone.
Ironically my old O2 contract would have been up around now, but I jumped ship to get a Hero 6 months ago. I doubt I can get an upgrade out of Oraneg now
Though I don't think the Desire is a massive improvement over the Hero, especially with an Android 2.1 upgrade coming to the Hero any time now, it looks like it pretty much fixes all of the minor negatives there were with the Hero compared to some other smartphones - Occasional slowdown, a camera flash, and an even more polished UI, and a higher quality screen.
Battery limitations are just a general design flaw of smartphones. But as someone else said, it's worth putting switches for Wifi/Bluetooth/Mobile Internet/GPS on your homescreen and switching them off if you don't need them at that time. If you have Email/Facebook/etc updating every 10 minutes that'll eat your battery, and GPS even moreso. I think the first 3 switch apps are on the phone already, you can download a GPS switch from the marketplace though.