The red admiral is a very widespread species. It has trouble surviving the winter in the UK (although some do) and so the population is boosted by migrants from mainland Europe. It is indeed a strong migrant, it has been recorded flying at night and also in sub zero temperatures.
The larva of the Red admiral feeds on Nettles, they tend to be fairly solitary in this stage of their development. For defensive purposes they use silk to close a leaf which they can then feed on in relative safety. The picture left illustrates this, it certainly looks formidable to me.