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: 
I'm in the same situation. I've resorted to creating a copy of draft 1 that can be annotated but not otherwise edited, e.g. a PDF in Google Drive. Such files are easy to edit. Highlighting isn't so feasible in GD, although I have a friend who finds Edge very helpful for that. (Phones vary in whether they can take advantage of this.) I use the commute to read the work, decide what changes to make to a given page, then write instructions to myself as an annotation. When I have time at my laptop at home and I'm not exhausted, e.g. at the weekend, I see how long it takes to obey the instructions for a chapter, and then see whether I have the energy for another. You'll get through individual changes much faster during "round 2", so it won't necessarily eat up the weekend. Try it, see if it works.
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.