That is an interesting question. There is a background radiation from natural radioactive sources in the earth and space that you cannot avoid unless you shield yourself. But do not worry about this as it is not high enough to be dangerous. Radioactivity is measured in some strange units that can be quite confusing but here is a good page that describes the various units:
As for dosages: I know there are national geological surveys in the US that measure the natural radiation levels around the country, and I assume there would be something similar for the UK. The average is 8-15 rad/hr but I would search around the Internet for something specific. The sites I listed above will give you information on natural sources of radiation around the globe.
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