In the law I have read where "a bus" is defined it is usually defined as something like "a vehicle having 8 or more seats not including the driver".
(What it doesn't say is why this can't include folded up camping seats... But I'm sure it doesn't

)
So you can probably use bus lanes.
Anyone know the act where the con. charge is defined? There must be something where the exempt vehicles are defined.
I'm sure you've worked out that if there is congestion people who don't need to be there will go elsewhere, and people who do need to be there will need to be there whether they have to pay or not.
Making any vehicles (except maybe 2 wheeled small ones) exempt is an admission that the charge is nothing to do with congestion.