I bought a Viking Phantom Race off sports direct in February, I've never had a better bike for the price. It's a nice road bike, but it has lots of clearance on the frame so you could easily put some fatter tyres on it (700x28C, probably) to make it more comfortable for tow-path commutes. It's around £130
iirc. Also, their after-sales service is excellent - a couple of weeks ago I was biking home and heard a horrendous crack, stopped up and found that two rear spokes had sheared at the hub. One email to their customer service team, they called back, asked what had happened, and two days later an entire new rear wheel turned up in the post.
As to frame sizes, I have a 30" inside leg which is pretty damn short and I tend to ride 20-21" frame bikes. What you're most likely to find an issue is the stretch from seat to handle bars rather than from seat to pedals. But ultimately, you can't really tell unless you go to a bike shop and try a few things out, see what's comfortable for you. Ideally a local independent shop is what you want, but an Evans or even Halfords should be willing to let you sit on bikes and see how they feel.