Usually importing equipment
to the UK simply means filling in a Carnet form, and ensuring that customs stamps this form on entry and exit. However, because the 1962 Carnet agreement with India only covers goods for exhibition purposes, and not film equipment, UK customs are currently insisting on a deposit being lodged with them for the duration of the shoot. There are ways of avoiding this, such as using a bank guarantee letter.
It is best to use an import agent if you are bringing in equipment from India, Dharma Productions and GS entertainment used the services of Kay Express. (link to Kay's site) Excess baggage can incur large expenses, and often the excess baggage is simple equipment, things that can be hired in Britain for a fraction of the cost.
Getting visas for your crew is a time consuming process, as the British High Commission needs enough time to process the application. You do not need a work permit to film on location in the UK. Procedures are different in other countries.