Most circuits will allow you to practice without a licence but most events in the UK require a licence issued by the Motorsport UK (formerly MSA) https://www.motorsportuk.org and most circuits belong to the Association of British Kart Clubs (ABKC) http://www.abkc.org.uk .
Both of these websites provide valuable information on how to get started in Karting.
If you need to call Motorsport UK regarding your licence the direct line is 01753 765050
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