Traveling to UK - Colombian Passport

[motu 5]

We are traveling to India this November and my wife is from Colombia - so she has the Colombian passport and her green card. Our travel involves flying to London and from there to India (and back the same way). It seems she will need to pay a Transit visa to do this. My question is - Do they charge her for the Transit visa twice - once going and then while coming back? We will be gone about 6 weeks only.

If yes then they will end up charging her 44 + 44 or 88 pounds ($92 + 92 = $184) -

A visitor's visa to UK is 63 pounds ($132);

Can she just get a visitor's visa and save the extra 15 pounds - ($52 or 2 bottles of Chivas)?

We would like to take the metro to Piccadilly and maybe take a city tour (on one of those double decker buses). Our layover of about 5 hrs (and 6 hrs on our return) makes it possible to take a couple hours of sightseeing and still make it back in time for our flights. Has anyone does this kind of thing? I did it on my last trip - but that was prior to 9/11 and I had a US passport and could sail through to the metro without spending much time in the airport - my layover was 10 hrs then.

Any thoughts, ideas?

Thanks