There are good answers but there's something they don't address.
While you certainly can do this, so long as each candidate is treated equally (and, if possible, you have someone else with you), there's a risk it'll achieve the opposite of what you want it to achieve.
- Moving part of a job interview to an informal setting doesn't stop it being a job interview. It looks like you're trying to create an informal social situation where you and the other person can get to know each other naturally. For the candidates, all the pressures will still be exactly the same, and they will still be trying to impress you, choosing their words carefully, preparing what to say, etc. There is a power imbalance, they are there to be hired, this can't be glossed over. They'll be in "job interview mode" while trying to also apply a veneer of informality. Which leads me to...
- You risk improving the chances of schmoozers and natural boasters, while reducing the chances of quietly competant candidates - which I believe is the opposite of what you're trying to achieve. You're creating a very unnatural social situation where someone has to present a polished version of themselves (because if they don't, they'll compare unfavourably with those who do) at the same time as faking informality. That's quite a challenge for most people, except for the kind of person who has spent their entire life turning informal social situations into opportunities to show off a polished, exaggerated version of themselves. That's probably not the kind of person you want to hire - but they'll be the ones who feel right at home here.
- You risk making people feel less comfortable and less able to be themselves simply because the situation is so unfamiliar - which, again, I believe is the opposite of what you're trying to achieve. No-one likes job interviews, but most people have experience with them, and know the score in terms of what is an isn't expected. People will understand that this is somewhere between formal interview and informal socialising, but they won't know where. What sort of jokes are appropriate, if any? What topics of chit chat are appropriate? How much work-related conversation is expected? What to wear? If it's the same day as the formal interview, will they be judged as sloppy if they remove their tie? Will they be judged as stuffy if they don't? Expect non-schmoozers to rely on you to set the tone even more than in an interview.
- It introduces lots of variables unrelated to ability - and regular interviews are already bad enough for this. Many people find the noisy hubbub of a busy cafe distracting (but would have no such problems in the office of a small company). Many people get a little self-concious about how they look while eating and might spend the whole interview worrying that they look like that over-used photo of Ed Miliband. Anyone with dietary issues will worry about appearing fussy or unreasonable. Many candidates will wonder if you're judging them on their menu choices. Candidates with jobs may worry about a current colleague seeing them in this public place; candidates without jobs may worry about the price. Etc etc etc.
If a candidate is uncomfortable, awkward and distracted, and seems difficult to talk to, how will you know which of these is the reason:
- They're not a good fit, you should hire someone else
- Their crazy ex from 3 years ago was one of the barristas, and they can't remember if he/she knows they're still in town
- They kept overhearing distracting snippets of the person on the next table graphically describing his recent health problems (something you, luckily, never noticed)
- They didn't realise that sandwich would contain anchovies. They hate anchovies. Who the hell puts anchovies in tuna mayo? But they don't want to appear ungrateful, or fussy. Gotta force it down. Mmm, anchovy mayo. Sorry, what were you saying?
- That person at the bar looks like Jane from accounts. S##t is that Jane from accounts? Why is she in this part of town? There's that company her team works with... are they based here? She's friends with Erica from HR, was it Erica who heard me say I was using this day's leave to re-paint my kitchen? S##t she's looking this way...
Etc etc etc.
Trying to create a more informal tone in an interview where the candidates' true personalities can shine through is an excellent aim, but unfortunately, creating a potentially awkward, unfamiliar type of not-exactly-an-interview scenario risks making the problem worse.
Instead, focus on setting an informal, friendly tone in the interview itself. A good interview by a good interviewer with a suitable candidate can naturally turn into a comfortable conversation where you can learn about what your working relationship will be like.
You're also lucky that in UK working culture, any work-related conversation can have any amount of off-topic friendly chit-chat added to the start or finish.
A little friendly chat at the start to set an informal tone (but don't expect them to be very open yet), searching questions in the interview that allow the candidate to talk about the ins and outs of how they work, which you steer into being as close to a natural conversation as possible, then, more friendly chat as you walk them out the building, which should now be much more open and familiar.