Some like Frank Pickles will do full car value cover for a little bit more, that takes away any guess work as regards future values etc.
Be careful with the first year new car replacement, I went through that recently for my Dad and its full of clauses. Main issue is if the model has been updated or increased in cost, this happened to him with a very minor upgrade and name change for the model. That meant insurers could have reverted to their standard clause of market value which was at least 3 grand less than needed to replace the car even only at 7 months old. Big fight ensued including insurers wanting to supply a new car via a broker rather than his local dealer who he had a good relationship with as regards servicing, collection of car etc.
Thats only a small part of the detail but my advice is to always take GAP for the full term, it doesn't cost much more to take 4 year cover rather than 3 therefore your car is fully covered from the word go, means if your insurers wiggle on the new car replacement you still have GAP to fall back on.
Also consider replacement insurance rather than return to invoice, they don't physically replace the car but rather give you the money to replace with a brand new car. In say 3 years there could have been quite a price rise and if you negotiated a discouht first time around you may not be able to get that discount again on a new car.