It seems the answer is that we don't know if it's spread yet. To quote the ECDC's briefing, "However, most EU/EEA countries sequence much smaller proportions of virus isolates than the UK, so ongoing circulation of this variant outside of the UK cannot be excluded". https://www.ecdc.europa.eu/sites/default/files/documents/SAR...

Basically, the UK is just better set up to detect variants like this than a lot of other countries. The other possibility is that even if this variant hasn't spread yet, there are other variants out there with similar properties that just haven't been detected due to the lack of widespread RNA sequencing. South Africa apparently has one that's similar but unrelated.