Recommendation for panniers to carry a laptop for daily commute That's available in the UK I've seen this question about Panniers, but as far as I can tell, the Arkel Panniers aren't available in the UK.

Does anyone have any recommendations for a good laptop holding pannier that converts into a backpack or a messenger bag style (I only don't want to wear it as a messenger bag while cycling), and is also readily available in the UK? 

I don't want to wear a messenger bag while I'm cycling, as my issue is that my current bag is making my back sweat like crazy.
You could always get a laptop sleeve and slide that into a big enough pannier.

Timbuk2 has a couple pannier options that both have an included laptop sleeve and are designed to be easily carried off the bike. One converts to a messenger bag, the other more of a "shoulder bag". Looks like Timbuk2 has some of their stuff sold retail in the UK, and they also do direct orders with international shipping. I don't have either bag, but I do have a laptop messenger and a couple regular messenger bags from them that I've been happy with.

Their site is a bit of a pain to provide links to, but looks like Basil has a similar kind of laptop messenger pannier. Their site shows plenty of distribution in the UK.

Ortlieb's rear/larger panniers have an "Office" and "Downtown" line that both seem to be designed to accomodate a 15 inch laptop (via a sleeve/insert you purchase separately). Two different mounting options.

What I actually do when carrying the laptop on my bike is either bungee a laptop messenger to the top of the rack or stick a (well-padded) sleeve into a pannier