In the mythologies that are there for the period in Scotland and England before the advent of Christianity, is there any mention of any practice of human sacrifice for religious rituals?
Yes, we have two main sources from the Early Roman Occupation era:
Strabo, Geographies (64-21 BCE):
The Romans put a stop both to these customs and to the ones connected with sacrifice and divination, as they were in conflict with our own ways: for example, they would strike a man who had been consecrated for sacrifice in the back with a sword, and make prophecies based on his death-spasms; and they would not sacrifice without the presence of the Druids. Other kinds of human sacrifices have been reported as well: some men they would shoot dead with arrows and impale in the temples; or they would construct a huge figure of straw and wood, and having thrown cattle and all manner of wild animals and humans into it, they would make a burnt offering of the whole thing
Julius Caesar, De Bello Gallico (44 BCE):
All the people of Gaul are completely devoted to religion, and for this reason those who are greatly affected by diseases and in the dangers of battle either sacrifice human victims or vow to do so using the Druids as administrators to these sacrifices, since it is judged that unless for a man’s life a man’s life is given back, the will of the immortal gods cannot be placated. In public affairs they have instituted the same kind of sacrifice. Others have effigies of great size interwoven with twigs, the limbs of which are filled up with living people which are set on fire from below, and the people are deprived of life surrounded by flames. It is judged that the punishment of those who participated in theft or brigandage or other crimes are more pleasing to the immortal gods; but when the supplies of this kind fail, they even go so low as to inflict punishment on the innocent.
This practice possibly continued during early Christianity. Medieval Irish Catholic texts mention the early church in Ireland burying sacrificial victims underneath churches in order to consecrate them. The most notable example of this is the case of Odran of Iona who volunteered to die and be buried under the church of the monastery of Iona.