  Croydon is a London Borough and is in the county of Surrey. Croydon is a town and so has a council. I live in Sanderstead, which is part of Croydon. Nicki lives in Hamsey Green, which is part of Croydon. Warlingham School is outside Croydon (by about 5 metres) and so is part of Surrey County Council.
That is why we get Surrey ambulances at school and a mix of Surrey and MET police. However, northern Croydon stops being Surrey and is part of the county of London, although never written like that. The City of London is actually quite small and covers between Westminster Bridge and Tower Bridge (or there about). The Queen, for example, does not live in the City of London, but lives in London. The centre of London, such as Westminster and the City is very different. London then technically speaking becomes a county and Westminster is a City within London, hence the signs saying "City of Westminster".
The towns around London, within Greater London, are part of the counties they are in, but the MET control the whole of Greater London. However, Thornton Heath, which is in Croydon, is in the Surrey part, but it has a London postcode: SE19. However, other parts of Thornton Heath are in the Surrey part of Croydon and have CR7 for their postcode. Most of Surrey has Croydon postcodes; those living in Warlingham and Caterham with CR6 will know.
London Gatwick airport is quite far south in Surrey, but is London's second airport, with Heathrow being first. Gatwick is confusing because it spans Surrey and West Sussex - the airport being in the latter. There are also constituencies to worry about as well. Warlingham School is in Tandridge, with Croydon to the north and Mid Sussex to the south. This is how the governments look at their national statistics. However, the constituencies within that are broken down even more. I get BBC LDN for my local news, as I receive cable from Croydon and terrestrial from Crystal Palace (which is another town, although not very big, but is part of Croydon). People living in Tandridge should receive BBC South East as their local news  but I do not think any of us live that far to reach that news. According the BBC and the office for National Statistics, Croydon is in London and Tandridge is in the South East. My address is officially: Sanderstead, South Croydon, Surrey  because, on a map, that is exactly where I am. There. 
