"I tether my laptop to my phone. No need for a dongle, I just turn my phone into a wifi hotspot." Some networks don't support / allow tethering, or charge a pretty steep amount (25 quid a month and up), which is a bit of a luxury if work won't refund any portion...
First question - what route is the commute (or do you see anyone 'computing on the move' so you can ask them all what network(s) they use). Check as many people as possible, and where they use/ fail to access their network. Be aware that their experience is not guaranteed on different devices, and even may depend which side of the train they sit...
Options may depend on what gives best coverage on your journey, or perhaps at the end nearest end of journey... you could collect any email overnight and then respond 'offline' and send shortly before arriving at destination station.
The 'ongoing costs' might be moderate to high, depending on whether you buy a laptop / netbook / mobile. Would you really prefer to spend 500 on a mobile for low cost mobile contract, or spend 15-20 quid a month and get the phone included?
There are some mobiles with keyboard built in. Not that many, and few of those with bigger screens as far as I know. The ones I have seen are mostly Android
But until you know what network(s) work either all the way or near the end of journey, it's difficult to predict best option.
Indeed - would a 200 quid netbook without 3G be workable, if you could download your mail at home and send it on arrival at the office (assuming they have no block on BYOD {bring your own device}) ?
A 3G netbook would have ongoing costs. A netbook you could use on wi-fi at fixed locations would have none but the initial outlay.
If you're terminating your journey in London, there may be some T-Mobile/ Virgin Mobile deals which would provide internet access via VM's wireless at some Underground stations (I'm 200 miles away but have 'access' in theory, if I ever re-visit London, because of my T-Mobile contract).