Thatcher cut taxes to stimulate the economy, all the while raising defense spending. This bit about being the world's policeman doesn't really fit. Besides, when the British Empire was at it's height it was really raking it in, what with mercantilism and all.
The Empire wasn’t what made Britain wealthy. Given the expenses incurred in garrisoning and running the colonies, it was probably run at a net loss. Being the pioneer of the industrial revolution and becoming the workshop of the world made Britain rich, just as it did Germany later in the 19th century (despite its minimal colonial Empire) and the US, and now China....