How are property taxes calculated in [...] London?

Properties are assigned to a tax band based on the tax office's assessment of the house's value were it to have been sold in April 1991. Bands vary from A (less than £40,000) to H (more than £320,000).

The council tax ratios between the bands are fixed - so a band A property always pays one third the price of a band H property. The band a house is assigned to is essentially permanent, although you can appeal if it's not in line with equivalent properties in the same area.

This provides certain desirable properties - more expensive properties pay more, but your bill doesn't go up just because house prices in your area go up.

Disadvantages are your tax bill can still go up - just it goes up for everyone by the same percentage; if an area has become much nicer (or less nice) since 1991, the council doesn't make any more (or less) money; and valuations based on a government employee's estimate of a hypothetical sale don't have the idealogical purity needed to really get the support of voters.