You are not only outside the JLR corrosion warranty, you are also outside the 6 year period generally covered by various Sales of Goods Acts and subsequent legislation and case law. If you had attended to this matter within six years you would be in a strong position. As it is, you aren't. See: www.honestjohn.co.uk/faq/consumer-rights/

My advice is to either take the 25% goodwill discount or, if you can get the work done for less than the Jaguar estimate minus the discount, take that route instead.