The links given above tell the story, worrier.
Don't panic!
Since an amendment in 2010, if more than 25% of the roof covering is to be replaced, then Building Regs approval is needed. NOT Planning Permission.
PP only comes in when a roof is altered/extended etc. Pretty obvious if you think about it. For a simple re-cover (assuming like for like slates/tiles), then apply to your local District Council (Building Control Dept.) for a "Building Notice."
No plans or drawings needed. The Building Control guy will simply take a look at it to ensure a few things are done......
1 Simply, that the job is being undertaken correctly (good tradesmen)
2 The roofspace is properly ventilated (no worries there - modern breathable roofing felt takes care of this automatically.)
3 The loft insulation is improved to modern standards. (just a matter piling in more fibreglass loft-roll usually.)
For your fee, you will have the reassurance that it's a good job, and also a "Completion Certificate" to show future nervous buyers.