Whaddya mean you're gonna deport me???
Yes, my friend, it is true - you need a visa to get into India in the first place. So best give it a bit of thought before going, make sure you get one in time. This will all depend on where you live, so best get local advice, and do remember to make sure that you have a passport valid for six months after you intend to leave India.
The Ministry of External Affairs site offers general advice and also has links on its Missions page to Embassies and Consuls around the world.
Now, perhaps it is only me that finds it strange but the eagle-eyed amongst you may have noticed that there is no mention of any British Embassies or Consuls there! Fear not my fellow Brits, for here is the High Commission of London website on visas. Best course of action would seem to be to turn up in person (with all the required documentation) at the High Commission of India in London, or the Consulate General of India which has offices in Birmingham and Edinburgh. There are also a number of visa 'surgeries' held at various locations throughout the UK during the year, check the website for details.
Most people will be looking for a short-term tourist visa, but all the info can be found on the High Commission website re visa types, application procedures, fees and opening times so I won't duplicate here. From what I read they will usually give you a short-term tourist visa for 6 months from date of issue, so I'm not planning to apply until November this year. Looks like you can get your visa there and then, but I'll update once I have done this part and let you know how I got on.
Also found out about this passport scam, mostly aimed at NRI's (Non-Resident Indians) but I think everyone should be wary. Just remember, truth and honesty are not always a protection in India, the police and officials can often be corrupt and a little (or a lot of) baksheesh can be the best way out of a situation.
Another good point came from this post on the Indiamike site. In brief, it is not usually possible to extend a 6 month tourist visa, except in an emergency which might buiy you a couple of weeks. What you need to do is leave the country and apply again - Nepal is recommended for ease of travel, and even an overnight stay is sufficient.