BritBrit introducing tourism visas is a very bad idea for a number of reasons.
Visa requirements would harm British tourism and business - report on economic impact here ec.europa.eu/DocsRoom/documents/13317/attachments/1/translations/en/renditions/native
The EU has a block visa policy, so if Britain were to introduce visa for EU citizens, in return Brits would be ^required to apply for visas for visits to any EU country. Applications need to be made at least 3 months in advance, normally at a consulate, with fingerprints provided from all applicants, proof of travel insurance and reason for travel, and €60 fee.
Now, you may say, this is all worth it because security is priceless!
Except that short-term visas would not make anyone safer.
To screen out potential terrorists, you need information. If the information comes from the British side (say they've committed a crime in Britain before), this is already known by border security agents, and they will already be stopping the person at the border the way things are currently set up. If the information comes from the EU (kind of likely if we're talking about EU terrorists), then Britain relies on information coming from other EU countries, mostly through the Schengen Information System and the European Criminal Records Information System. Which the UK has access to as an EU country.
If the UK were to leave the EU, it would have if anything less access to all this EU information about potential terrorists. The EU has not granted ECRIS access to any non-EU country, and the only non-EU countries with access to the SIS are Norway and Switzerland, both part of Schengen.
These are just a few reasons why it's all a bad idea, for a more detailed explanation see
eulawanalysis.blogspot.co.uk/2016/04/goodbye-cruel-world-visas-for-holidays.html