Question: I've just started editing my novel. 

For the drafting I had done most of the work on a phone, as I wanted to make the most of the daily commute. This worked well enough and the first rough draft is done. 

However, I'm finding the same approach just isn't as strong for editing as it's not easy to highlight and remove words or find things quickly on a phone. 

Does a portable computer exist that would be up to the task? A tiny laptop could work but I currently commute by bus (not train) so there isn't exactly a table or other surface I could rest a computer on. 

My other option is just working weekends, but I'd rather avoid this if possible. 

Can any writers recommend any suitable tools?

If not, how did you keep productive during the editing stage?

Answer: 
Tablet, with tablet pen is the best mark up tool, period. However, turning a marked up document into a new draft needs two panels and that won't ever work well in transit. You're going to need to find time outside of your commute for revision & editing, but you can reduce the task load.
To add to what others have said: 

If you choose a tablet:
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* the touchscreen keyboard will be much less ergonomic compared to a real laptop. If you are the kind of person who may be discouraging by that, you can get a wired or Bluetooth portable keyboard.
* a tablet + accessories setup will be more fiddly than a laptop. With a laptop, you will only have one device that you will have to remember to carry and keep charged. With a tablet and keyboard, you will have to keep two charged, and connect/disconnect them occasionally.

If you choose a laptop:
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* they are more tempting to leave behind unattended because of their weight, "just this once, for only a minute". That can get them stolen more easily than with a tablet, which you can train yourself to pick up whenever you leave the room.