To answer your question, maybe the amount of money a council needs depends on how densely populated a region is? So a council with lots of people living in it needs more money to provide more services? Perhaps Westminster isn't as densely populated as the area you live, so they don't need as much money?
Actually, it's the opposite. It's more expensive to deliver services in a sparsely populated area. In a rural area, there are a lot more miles of roads to be maintained, more libraries, more schools (although the schools will be generally smaller), more fire stations, the council will need more dustmen to get all the rubbish collected because of the distances they have to travel and so on.
The London borough where I used to live had 7 libraries. The county where I now live had 30, for a similar number of people (they have had to shut several to make savings). And they had a mobile library service to serve isolated villages. Every year, the council has to cut hedges on all the rural roads, not something that happens much in an urban area.
"Sparsity" is one element in the complex formula used by central government to allocate funds to local authorities through the revenue support grant.
This is reflected in our council tax, which has been in the top 10 most expensive in England and Wales for 20 years or more and was the most expensive for several of those years. We have a band C property, and this year, our council tax was over £2,000. From April, it is going up by another £120 or so.
To add insult to injury, the project I work on is funded by the county council. They are slashing our funding by 50%, so I may well be made redundant. It's going to be bloody hard to find that money out of one income!