with regards to building regs thats up to you I was having a discussion similar to this with a BCO a while ago he basically said don't worry about it how are we to know you replaced your roof, we would only really get involved if we were on site for other reasons such as the roof being replaced as part of other works.
You should be able to find plenty of tiles for a 17.5 degree roof, the problem with increasing the lap on the tiles is they won't sit properly over each other if you increase it too much i.e the exposed edge of the tile will be pushed up and this will defeat the object
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