I think he said Working Aisle which to me means not the Right angle aisle.But the minimum aisle that I can use with my configuration of truck and my load (pallet whatever) to enter an aisle and turn 90 degrees to put the load away and exit.
The big factor here after we have sorted through all the technical specs and it gives you a number that is made for an Olympian Fork Lift Driver who has trained for six months for the test,is to look at your own operators. Basingstoke is not too bad but here we give them long service medals for showing up for three weeks. So you have to start adding on to that aisle dimension for the experience you may have in your drivers, or the consistent level you may reasonably be able to train them to with your labour force.
Then make a judgment based on the speed of your operation. Grocery warehouse being the worst where all has to be out by 4 a.m. and a regular finished goods warehouse with a steady flow of orders being the best.
Lastly where are you geographically with regards to labour and rack damage. Last time I was in Mexico I saw one forklift going down the aisle preceded by two helpers, you in the UK probably have the worst situation and want as little rack damage as possible within reasonable limits.
Silly as it seems I have had customers put 2" x 4" long lengths of wood or similar steel down on the floor and practiced with their drivers. A very illuminating exercise.