Europe is "metric", meaning it is uses the IEC 60228 standard gauges. Common wires at local store thus are 0.5, 0.75, 1, 1.5, 2.5, 4, 6 square mm. In my experience, the 0.75 to 2.5 sq mm range is carried by even small stores.
Historically, the British use[d] their own SWG but this changed roughly when they joined the EU; Brexit will apparently not change anything in this regard... for the subtle reason that the EU-wide standardization body in this area (namely CENELEC) technically is not an EU institution... so the UK is still a member of it after Brexit.
The colors of wires used in European power cables (e.g. brown, blue, red/green) come from CENELEC HD-21 and HD-22 documents, recently merged as EN 50525. The same standard specifies the types of common power cables that you see in stores, e.g. H05VV-F etc. (Note that the latter CENELEC standard begins with a 5, not a 6, which means it is a regional European standard, not an incorporation of a IEC standard, like the metric gauge one.)
Finally, "home electrical wiring" involves much more than wire gauge or the color of wires. Generally, there are two sets of standards those of (US) NEC and those of IEC. Legrand has a map of roughly where they apply (reproduced below). Going into more detail is far too elaborate for this Q&A format; for further information, refer to the document I extracted this map from.