Being islands, the British Isles are not exactly easy for foreign migrants simply to walk into. As a result, if your ancestors were in Britain in 1900, most of theirs were probably there in 1 CE, also. There are, however, five historic conquests/migrations that you must account for.
England , Wales, and southern Scotland (but NOT Ireland and northern Scotland) were under Roman rule from the 1st Century to 5th Century (some later), with significant Roman settlement. Thus, if your ancestors were in these parts in 1000 CE, their ancestors could have lived hence some inhabitants of these areas could theoretically come from ANYWHERE in the Roman world in 1 CE (Indeed, Roman emperor Constantine's mother was British.).
When Roman rule collapsed in the 5th century, Southeastern Britain was invaded by the Anglo-Saxons, an assortment of Germanic peoples living in the Low Countries, Frisia (coastal West Germany), and what's now Denmark. They conquered and intermarried with locals, giving rise to the English nation. Thus, if your ancestors were English and lived in England in 500 CE, their ancestors lived in England, Denmark, Frisia, and the Low Countries in 1 CE. Note: The Welsh are descendants of the native British that managed not to get swamped by the invading English. Thus, if your ancestors were in Wales in CE 500, their ancestors could have been anywhere in Britian in CE 1, but probably not on the European mainland.
Not wanting to miss out on the act, Irish raiders/settlers, the Scots, invaded northern Britain at the same time. If your ancestors were in Scotland in 500 CE, then theirs were both there and in Ireland in 1 CE.
Starting in the 9th century and lasting into the 11th - the "Viking age", Scandinavians from Norway and Denmark began to raid, then settle in Britain. They eventually took over and established permanent settlements in North England, Wessex, and the Midlands, where there was widespread intermarriage. Indeed, for a brief interval, the Scandinavians held all of England. If your ancestors were there in 1490 CE, it's safe to assume that they had ancestors in Norway and Denmark as well in 1000 CE.
Ireland is a special case. During the Viking Age, Ireland was completely conquered by the Scandinavians, who founded all of its major cities. If your ancestors lived in an Irish city in 1000 CE, you may safely assume they had ancestors in Norway and Denmark in 500 CE.
1066 CE. The final invasion is that of the Normans - culturally fankified Scandinavians. They were French-speaking and formed what ended up being the aristocratic class of England. If you have aristocratic English ancestors, or English ancestors with French surnames, you should assume that their ancestors were living in Normandy in 1000 CE and in western Scandinavia in 500 CE. 1066 CE. Southern Scotland became a refuge for nobles fleeing the Norman invasion. If your ancestors lived there in 1490 CE, it's possible they were from farther south in 1000 CE.
Starting in the 16th century, the English conquered and controlled Ireland. Northern Ireland saw the immigration and settlement of many people from Scotland. If your ancestors were Irish Protestants in 1800 CE, their ancestors probably lived in Scotland in 1490 CE.
Recently England has received immigrants from all over the world. Your family history should help you sort out whether your English ancestors are, in turn derived from India, Africa, the West Indies, Ireland, etc.