Many nationalities are now eligible to apply for visa on arrival. This e-Tourist Visa (eTV) is available for holders of passport of some 44 countries, including the UK. It’s valid for 30 days and costs $60 (£39).
See : https://indianvisaonline.gov.in/visa/tvoa.html
You have a window of 30 days in which to apply for your e visa (eTV). You can set your arrival date up to 30 days after you apply on line, but you must apply at least 4 days before you plan to travel.
E.G If you are arriving in India on 1 November, the very latest you should apply is 4 days before that date: 28th October, and the first time you can apply will be 30 days before that: 28th September.
These eTVs are for single entry only and you can apply for a maximum of two visits in any calendar year.
STAYING MORE THAN 30 DAYS?
If you plan more than this then you’ll have to apply for a visa from the appropriate agency. In many countries this is VSF Global www.vsfglobal.com . You will need to download a form and take or send it to the appropriate agency office in your country.
In the UK apply by post or in person to VSF Global (agency processing visas on behalf of the Indian High Commission). Download a form and pay on line at www.vfsglobal.co.uk/india/ .
It will take 3-5 working days for UK passport holders to get your visa if you apply in person; and 15 days, plus transit times, if you apply by post. In certain cases the processing time is much longer: e.g. for people of Pakistani origin, and for non-UK passport holders residing in the UK for less than two years. Please check the website.
VSF Global have 14 Centres in the UK including in London, Edinburgh, Birmingham, Belfast and Cardiff.
In the USA the please see: http://www.in.ckgs.us/
Otherwise, please contact the Indian Embassy in your country.