According to
http://www.proeuropa.org.uk/twelevereasons, the UK enjoys all of this thanks to the EU:
1. Jobs
Around 3.5 million British jobs are directly linked to British membership of the European Union’s single market – 1 in 10 British jobs.
2. Exports & investment
The EU buys over 50 per cent of UK exports (54 per cent of goods, 40 per cent of services).
Over 300,000 British companies and 74 per cent of British exporters operate in other EU markets.
American and Asian EU firms build factories in Britain because it is in the single market.
3. Trade
The EU negotiates trade agreements with the rest of the world. Outside the EU Britain would have to renegotiate trade deals alone. While the EU is the world’s largest market, a UK outside the EU would not be a high priority for other counties to negotiate a trade deal.
4. Consumer clout
British families enjoy lower mobile phone roaming charges, lower credit card fees, cheaper flights and proper compensation when flights are delayed or cancelled. These sorts of benefits could not be achieved by Britain alone.
13. Research funding
The UK is the second largest beneficiary of EU research funds, and the British Government expects future EU research funding to constitute a vital source of income for our world-leading universities and companies.
A Brexit could mean that there will be increased taxes on all of this to compensate from these lost advantages, which will surely translate to increased corporate taxes.