The MOT is a VOSA/Government test. They set the standard/legal charge for an MOT. No MOT test station (as they a licensed to do VOSA/government to do the testing) can charge more than the VOSA official fee.
To my knowledge (other than a VOSA site license if such a thing exists) the MOT station does not have to pay anything to VOSA for any MOT undertaken. A garage/MOT test station can charge less than the official VOSA fee. That is their decision and how they split the bill up (labour + paperwork) is I guess their own decision as long as the combined cost for the MOT Certificate does not exceed VOSA's official pricing.
Personally I always get my MOTs done at a local county council testing station as opposed to a garage/dealer.
Now most reputable dealers from who you bought your car will a) want to keep your custom b) keep your servicing custom etc. This means that they will service and check before MOT so there will/should be no surprises other than a PASS!
Many garages offering MOTs either do the MOT themselves (if approved) or use an approved MOT garage/testing station. These often tend to offer cheap MOT's, especially if a service is included. Some may also invite you with a low MOT cost but then get "picky" and expensive on possible/dubious remedial work required to pass MOT.
Council type MOT testing stations (I believe all local and country councils have to have MOT facilities) will do an MOT, rates are fixed at the VOSA official rate and in my experience they are very fair, honest and not picky! These council workshops have no repair facilities for the public (council vehicles only) so they are not looking for work. All they are looking for is unsafe vehicles. Point to note. They don't do pre MOT testing etc. so if your vehicle has not been serviced and looked after then it may fail the MOT.
Of course there are permutations and combinations MOT only, servicing only, MOT + service, etc. and all I would say/add is that everybody has to go with what they think/feel is right and then measure your experience and cost against the overall outcome.