How can I stay for eight months in London on a two year UK visitor VISA with multiple entries not exceeding 180 days.
You can only spend 6 months in the UK on any one entry.
There is no formal limit on the number of entries or their cumulative length but there is a rule that people on visitor visas (or visa-free visitor entries) must not try to "live in the UK" through "frequently or successive visits".
The result is that trying to exit and quickly re-enter to extend your stay is a risky move. The border gaurd can let you back in with a new 6 months stamp but they can also decide that you are spending too much time in the UK and/or that they suspect your activities in the UK are not within those allowed on a standard visitor visa and kick you out.
What do you mean, 8 months without going back to your home country? if it is yes then it's impossible. I believe that it is a total of six months in 12 so if you leave after 2 months then come back you still have 4 months to visit again. If you overstay then you will have a conflict going back to the UK or reapplying for an any UK visa. Read more information here