Not the answer I expected Spellbinder..
If they were here for about 400 years, I can't understand how they would one day say, 'come on, we're going home now' because Britain would have been home for many by then.
You've kind of answered your own question here! The concept of the Romans packing up, going home and turning the light off on the way out is an antiquated idea. Many scholars now see the end of Roman Britain as a gradual decline as opposed to a dramatic collapse.
The problem lies in what we mean by the term 'Roman' - it encompasses so much and by the 4th century meant a hell of a lot more than someone from Rome, indeed the majority of Roman citizens would most never have heard of Rome! For instance, in the Confessions of St Patrick, we learn that Patrick thought of himself as Roman as anyone despite being someone from Northern Britain (most likely) and never stepping foot out of his own front door until his ventures into Ireland.
For Wirral, the first evidence for Roman interaction comes at least 100 years before the Claudian invasion of AD43. Numerous coins from Meols suggest trade with the Roman World at least 100 years before this which may perhaps be linked with the Cheshire salt trade. With the founding of Chester in approx AD78, Meols boomed as a trading centre, and it seems that Wirral may also have been home to craftsmen making distinct jewellery (known as the Wirral-type Brooch) and acting as a deep-water port for Chester.
Indications of further Roman settlement have been found in Irby where an Iron Age farmstead seems to have continued throughout the Roman period, and of course Leasowe Man (the oldest human remains found in Wirral) is of Roman date, though not necessarily a Roman!
Christianity was indeed a Roman introduction to Britain, but was at first no more than an eastern mystery cult to rival the numerous others such as those in honour of Bacchus, Mithras, Isis etc. Christianity first developed in towns (indeed, the term 'pagan' orignally meant someone who simply lived outside of a town) and so Chester is likely to have been an early Christian centre. The first British martyrs to be killed were called Julius and Aaron were meant to have been executed in Caerleon in Wales, but this is just as likely to have been in Chester which was also known as Caerleon (City of the Legions.
The most intriguing place-name in Wirral (and likely the oldest) is that of Landican which seems to represent a prWelsh rendering of Llan Tegan (Church enclosure of St Tegan) which would represent a very early post-Roman place-name and therefore suggestive of an early foundation church. Similarly the dedication of St. Hilary in Wallasey is incredibly rare and may point to an even earlier foundation of Christianity on the Island of the Britons (Wallas - British).
The Anglo-Saxon incursions into Britain in the 4-6th centuries forced Christian communities to the fringes of Britain, hence why our oldest Christian sites are found in Wales, Cornwall, Scotland, etc. Wirral sits firmly in this 'zone' and as such plays a part in this amazing story of an age that was anything but dark. Recent archaeological evidence is piecing some of these pictures back together so hopefully Wirral place in the Roman world will soon be able to be retold in more detail.
Just my 2 pence worth