Well originally they had no mains at all, just the 50VDC from the exchange batteries, but recently (last 20 years or less ) mains has been added to those that have had to support ADSL and G-Fast  equipment by getting the DNO to  run in a  line from the nearest substation, Now, as the fibre network is passive, power will not be needed, I suspect that most will not originally have had an adequate supply, or at least the fuse in the head will need changing. Equally  for mechanical reasons the DNO will have run in at least a lamp post level supply - which in many areas of the UK will mean it is 10-25mm2 even thought BT only needed 5 amps.
The bigger question will be the common one these days, of how much slack capacity is there on the DNOs transformers - round here fan kits have been popping up on substations and transformers outdoors in odd places, presumably as a far cheaper retrofit than a transformer swap.

Mike

Around here lamp-posts generally get supplied in at east 6mm² if not 10mm² or 16mm² (for probably a mix of reasons including mechanical robustness, fault protection reasons and convenience of connection to the larger mains cables). I doubt anything smaller was used for the cabinets, so cable size is unlikely to be an issue. A fuse upgrade if needed wouldn't be the end of the world.  I suspect most will be single phase though, which is OK for a 7kW charger, but I suspect most cars will want 3-phase for 22kW, which might be more of an issue.

The biggest problem I see though, is that all the cabinets around here are located at the back of the pavement - so I don't see how anyone could plug in an EV without running the lead across the pavement - which isn't permitted. They have to be moved, the re-using the old cabinet isn't really much of an advantage.

   - Andy.

Many of the green cabinets I can think of are also on road junctions.  Not the places you want people to be parking up to charge their cars.

I'm not aware of any car that can charge at 22kW single phase.  It's usually 3.6 or 7.2kW single phase, or else 11 or 22kW three phase.  A lot of cars can't actually charge at 22kW on AC, as they have been optimised for DC charging.

The advantage is only I suspect that there is already permission to plant a cabinet there,and as the wayleave has been granted and paid for, for however long in the future, so it might as well be used - though I agree about access - Mostly round here (Hants) they are tucked into hedges and so on not placed at the kerb beside handy lay-bys. The pelican crossing boxes are on the kerbside, but there is a very good reason you do not want anyone parking there either.

I suspect, like the 'charge point at every lamp-post' nonsense idea it will be OK in a few cases, but is not applicable as often as folk might hope - it sounds better if you say it quickly.

Mike.

For more info here's an E&T article from July last year 

and the recent article on the BBC

I am not sure there are many redundant street cabinets with 3 phase supply power.  Would be interesting if there was a list from BT.  Next question.  Are they roadside or pavement side?  Some are in hedges and all sorts.  This would mean a cable across the pavement which is technically not allowed but I understand that some councils will now allow a cable across the pavement(I think Portsmouth). What about PEN fault detection?  Is it safe to leave a vehicle next to the cabinet and still allow other road users safe space to go around?  Are the local substation and DNO/DSO transformers up to the task of the extra power load? 

As an edited afterthought.  Is the supply in the cabinet metered or unmetered?

It will be interesting to see how this project develops