13 amp is all that + cost of watherproof socket ~ £20 + VAT on top
The only variable is socket type and any type 2 will cost way more.
No. The type2 can load share to ensure that the total load across all the sockets doesn't exceed the supply available; 13A sockets will have to be given a supply big enough for all of them to be in use at once. As soon as you go above a small number of sockets, the cost of the supply and wiring is likely to dominate.
The installation of sockets for EV charging isn't remotely like installing a similar number of general-use sockets inside a building.
2 times 2-3-3.5KW is probably better than 1x7kw. for a work car park where people are there for 7-8 hours and most people live a lot less than 45 miles away.
7kW units with load sharing is the right answer (because the cost difference from fixed 3kW is trivial). There's a slight argument for 11kW units (on the basis that the early arrivals can fill up quickly, leaving more power available later on for others), but the extra cost probably isn't justifiable with the current mix of vehicle types. With a large installation, an average of much less than 3kW across all the sockets may well be sensible, though getting it down to the minimum possible requires that there's some means for cars to indicate their level of need and how long they are staying, which makes things more complicated.