Only about 10% of the UK economy is exporting to the EU, a larger poriton is exporting to non EU countries with the bulk of our economy being being intenral.
We also very importantly are a services based export economy - the only one in the EU like this and there is no proper free market in services as it is and that is where our massive position of strength is and why we are uniquely placed to make Brexit succesful if we had anyone who actually wanted to leave in charge. We run a trade surplus in services with the USA that we in turn spend that surplus on goods from Europe. London is the financial capital of Europe and no other EU country could even take that from us if they wanted to due to the fact that they completely lack even a fraction of the infrastrucutre to do so and it will take years to build up and a massive concrted effort of so many different organisations - it's not realistically going to happen as there are ways around it as it is with 'brass plating'. And that is not to mention that the regulations, 19% corporation tax rate, personal tax rates and potential financial transaction taxes being suggested by Junker that make the cost of doing business in the UK much cheaper than the vast majority of EU countries. Even Ireland who offer a lower corp tax rate of 13% are already pressured by the EU to bring it in line with the 25-30% tax which would be about a doubling of what it currently is. It would be cheaper for most firms to stay in the UK than moving and then paying the much higher EU tax rates even if there were tariffs.
But there is no way on Gods green earth the EU states wont do a FTA with us as their economies relies on us buying their goods - it would be utterly counter productive for their intersts. We are generally the second biggest customer after the US who are threatening a trade war as it is, are the EU nations really going to want to put additional and needless tariffs on their 2 biggest customers? We buy about 70 billion more than we sell to them. They know that no FTA means the near 100b we pump into the EU economy every year in purchasing power will see all of their good become more expensive and less competative than the rest of the worlds good. If we can buy the same goods cheaper elsehwere then we will and they will be having a massive drop in trade. When you buy so much more than you sell like we do that gives us huge leverage. They've already offered us a FTA that included tariff free access to all sectors about 2 and a half years ago - Baker and Davies were writing the speach to confirm that the destimation of Brexit had been agreed - Theresa May blocked it as her only goal is to Remain by the back door. If we ever managed to get shut of May and her cancerous Withdrawl agreement and left the EU under WTO we would immedietely trigger article 24 of the WTO which locks existing trade conditions for up to 10 years and the previously offered FTA would almost certainly be negotiated. The alternative is that we could remove tariffs on all imports we don't produce in this country or even remove tariffs on all imports and go full Singapore - it's up to us as we'd have control over our tariff scheules. Or Even if we do go along with the EU tariffs the money we would collect in import duties on goods coming in from EU companys the government could subsidise our UK exporters to the EU - not to mention we'd avoid sending 16b to the EU in tariffs we collect on non EU imports on top of the approx 10-12b NET we send in membership fees and another couple of bill in defence and foreign aid spending. So there are plenty of options to avoid hurting our 10% of exporters and massive opportunities to boost everyone else.
Our EU trade balance has shrunk by about 10% over the last 20 years despite it being tariff free while the none EU trade has risen by 10% over the same period despite having tariffs on it. The EU comission itself has admitted that 90% of all future economic growth will come from outside of the EU. If we only care about economics then leaving the EU has the most long term economic prospects.
Don't be so dramatic about 'destorying futures' - it's project fear nonsense - you live in the 5th biggest economy in the world you're future will be fine - we aren't going to leave the EU and turn into Venezuela. The top 3 economies in the world are not in the EU, 6 of the top 10 economies in the world are outside of the EU, 14 of the top 20 ecoomies in the world are outside of the EU. Most of those economies trade succesfully with the EU on WTO terms or have signed FTAs that eliminate most tariffs on goods - like the EU have already offered us. What makes the UK so unique that we can't possibly trade without being part of a political union when the majority of our history we did that and the majority of the world does now?
Our only issue with Brexit is our politicians trying to sabotage it - we have so many levers we can pull economically and so much leverage to make it work for us.