I'm currently working from home, but finding this too solitary, and also need a change of environment in general so I'm considering moving cities. I think this is a great time to branch out of my residence and work somewhere else, but I'm not really sure how to go about doing this. I work with free/open source software, so I think posting messages on various user groups might be a good way to research potential space.
Does anyone have any tips on where/how to find shared office/co-working space?
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Look for incubators as well. Here in Cambridge we have a few science parks and innovation centers which boast a range of services from desk rental (hotdesking) to small office spaces and fully managed incubator premises. As well as co-working, check out the incubator facilities and search for "shared office space [city name]". St Johns Innovation center is one example.
Some cities are just better than others for startups. I know you didn't ask this, but if you really have the flexibility of moving to any city in the UK bear in mind that some cities are really well set up to cater for startups. Cambridge has a variety of organisations like the Cambridge Network, Cambridge Angels, CHASE, CETC and others (all beginning with C). The Business Weekly reports heavily on startup and business stuff in this region. Added to that are the aforementioned science parks and business parks, the University and academic culture. If you are looking for motivation, inspiration, other like-minded startup founders a place like Cambridge would be ideal. Furthermore, not trying to sell the place or anything, we have a high concentration of software, high-tech and biotech companies here. If you can't get over to Silicon Valley, head here to the Silicon Fen instead :)
The point here is that regardless of whether you find Cambridge attractive or not, there is a big difference in startup-support that you may get from city to city. Definitely worthwhile doing the research. Whilst you'll find events like this in Cambridge, I'm not sure you'll find the same sort of thing in {insert random UK city name here}.
I worked at home for 3 years building my company then moved to co-working style space, then upgraded to a cubicle. The psychological benefits are incalculable. I still work at home too, but knowing that work and life are separate helps me do better work. There's something incredibly profound about being around others while working.
If at all possible, do transition to a workspace of some kind. As long as the environment isn't like a coffee-house/grown-up playspace, the benefits far outweigh the cost.
Not sure if by the time of my reply you already have your office? Anyways will tell you that there are many companies who provide fully serviced offices which include everything you mentioned in your post plus lot's of other options. All you need is to search for shared serviced offices and the location you need.
Look for "coworking" space. One site to start of is http://wiki.coworking.info/w/page/16583831/FrontPage, and that's is a directory of coworking spaces all over the world http://wiki.coworking.info/w/page/29303049/Directory (there are several in the UK).