The Met Police has told cyclists involved in the 'critical mass' procession to keep moving and keep south of the Thames.
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A Met Office warning covering much of England and south-east Wales is expected to remain in place until 11am.
A band of cloud and patchy rain is expected across Scotland and Northern Ireland, moving southeast as the day progresses.
Charity warns an estimated 30 child refugees a day are being expelled from EU countries after crossing the border from Serbia.