It is fairly standard that bus companies pay a wage while you're training.
(I'm not entirely sure, but I think that if you are in the training period of a job, then minimum wage law still applies)
The training wage is of course usually less than the starting driver's wage.
Most bus companies have pay increments over (for example) the first five years
In the late 80s / 90s, pay increments tended to be based on starting on mini-bus, going to midi-bus then to big bus (the exact size at which each was defined did vary from one company to another) but most now do pay increments based just on length of service. (often, if you join as an existing PCV licence holder, you by-pass one or two of the increments)
Likewise, there was a trend to start new drivers at a 'low cost unit' then it was a case of waiting for a vacancy at the 'proper' company - this is mostly a thing of the past. As is the situation you got in London when route tendering started that the (ex) London Buses companies tended to create new terms & conditions and wage rates for each route they tendered for.
I have heard of companies where it can take 10 years to be on the top rate.
Also be aware if you're thinking of joining a bus company as a trainee driver, most companies have a clause in the contract that if they put you through PCV training, and you then leave within X time (2 years is common) then you are required to pay them something towards your training costs. (basically, bus companies do not want to be a free driver training school for coach operators.)
Finally, be aware that choice of what route / rota you go on does tend to be based largely on seniority - this will vary from one company (and maybe one depot) to another, but generally you start as 'spare' then when there's a vacancy on a rota, there may already be a waiting list for it, and there may be (what used to be called) an 'old mans rota' which tends to be the longest serving drivers at that depot for which there's a long waiting list...