Who said we would instantly get an amazing FTA? Not me, of course there's compromise when agreeing an FTA. Great Britain has been historically pro-free trade, and it would depend on the governments in each two countries agreeing the FTA how far an FTA would extend. An FTA with the EU for example would have the drawback of higher tariffs on agricultural goods (mainly due to the French protectionist farming industry) yet that would be offset in that we'd then find ourselves free of the EU agricultural area - which is very expensive for the French reason - and able to trade agricultural goods with the likes of African nations as well as New Zealand much more easily, just as we used to but which we are forbidden to presently under EU law. Essentially cheaper food which over time as an added benefit might actually have the rest of the EU pile pressure on France to weaken its stance on free trade, especially with Africa which is a huge untapped market.
Financial services on the other hand, which we're best at, would be much easier to agree on in FTAs not only by their nature (financial services run on the basis of free trade) but also that most of the financial capitals are outside of Europe and have historic links with this country, as well as being in up and coming regions (the Asian Tigers, America, the States, India, China). Plus, the fact we're the top financial service country as well as being the 6th biggest economy in the world will also help us conclude other parts of the FTA: would the EU really want to purposely tick off and isolate itself from the world's financial capital and cut off all that lovely funding in gilts, bonds and banking over regulations concerning say car production? I don't think so.
On the uncertainty argument on leaving... that's exactly what we heard when the Euro was created, that business would move if we didn't join it, it was completely false. The fact is we're literally mad to be chaining ourselves to a 1950s trade bloc in decline whilst our Commonwealth family is growing rapidly.

