How does a UK multiple entry visa work?

I have a two year UK visa with multi entries, for 180 days. How are these days calculated? Is traveling to France or Spain taken as leaving the UK? Can I then enter the UK again or must I return to my home country (South Africa) before returning to UK to start another 180 days?
Best Answer
I see you have two very closely related questions. The first being what counts as exiting the UK, the second being, how often can you enter the UK with a multiple entry visa?
The answer to the first question is quite simple. You have to leave the country through an immigration check. This is as simple as crossing the Channel by boat. You can also fly to any other country. It does not have to be your home country.
The second question is slightly more complicated. For more information about this, see Doc's detailed answer over here: https://travel.stackexchange.com/a/18947/36405. A quick summary: A visa is issued for a specific purpose, like vacation, visiting family/friends or business. It is not for living in the country. So-called visa-runs are certainly possible (going to France and immediately going back), it is against the purpose of the visa. Use your visa for it's intended purpose.
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How do I get a multiple entry visa for UK?
You can apply for a visit visa by making an application from outside the UK....How to apply for a multiple entry visa?How does a multiple entry visa work?
Multiple-entry visas are a type of visa that allow the holder to enter the destination country more than once. This means that the traveler can make multiple visits using the same visa during its validity period. Some multiple-entry visas and travel authorizations allow a specific number of visits.How many times can you travel with multiple entry visa?
While valid, a multiple entry visa will let you travel to Canada for six months at a time as many times as you want. It will be valid for up to 10 years or one month before your passport expires, whichever is shorter.How many times can you visit the UK in a year?
There is no limit on the frequency of visits or specified maximum period visitors are allowed to spend in the UK in any 12-month period. However, if you spend more time in the UK than in your country of residence the Immigration Officer may have grounds to believe that you live in the UK.UK 10 Years Multiple entry Visa|| UK Refusal Got Visa || London Visa || United Kingdom 10 Years Visa
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Answer 2
I have a two year UK visa with multi entries, for 180 days. How are these days calculated? Is traveling to France or Spain taken as leaving the UK?
Yes
Can I then enter the UK again or must I return to my home country (South Africa) before returning to UK to start another 180 days?
Visitor visas are not for living in a country.
Some countries have hard and fast rules on total length of multiple visits (for example the famous 90/180 rule in schengen). Others like the UK and the USA give the border gaurds more discretion. It's up to you to convince the border gaurd that you are a genuine visitor and not someone lving in the country while claming to be a visitor. The more your travel profile looks like someone living in the UK (i.e. long periods in, short periods out) the harder that will get.
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