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Morning Glory says, whilst a deep or lidded roasted pan will cook the chicken and potatoes, they won't get properly browned....which is after all the whole point of roasting.
When you cook meat in the oven the fats are aerosolised (ie. dissolved in the steam that comes out from the meat) and they then settle on the walls of the oven. That's what makes an oven dirty. So unless you only ever cook with the lid on (which as already mentioned, doesn't work for roasting) then I'm afraid the oven is not going to stay much cleaner.
I can't believe there are no oven cleaning companies in the whole of Liverpool....even our relatively small town has several. Checkatrade is only one listings service (and quite a niche one because its expensive for the trades to advertise on it) so here's another one, which has 7 pages of oven cleaning companies in the Liverpool area:
Oven Cleaning near Liverpool | Get a Quote - Yell