I had a few hours spare today, so I took the front wheel off to measure it accurately. The outside width is 26mm and inside width is 19mm.

I've basically narrowed down to 2 tyres due to budget and being available on ebay, Schwalbe City Jet 26 x 1.5 or Kenda Sport Semi Slick 26 x 1.25. What do you guys think is best?

Either way, I'll have to get new inner tubes as the ones I have are 26x1.9/2.125, so I'm assuming they won't fit a narrower tyre.