1. Irish history shows that, for most of the "800 years of English Oppression", it was one Irishman oppressing another. For most of that time, right up to the 19th century, Ireland had its own Parliament where Irish Lords made Irish laws for royal approval by the King of Ireland. It was due to the Earls, and the clan chiefs with whom they were often feuding, that the English King was invited into Ireland, and such peace as there ever was on that poor island was due more to English rule than to anything else. There were periods when English oppression was severe - Cromwell is a notable example - but at most other times, the violence was due to the inability of the Irish to live peacefully together, and English troops were forced to quell the not infrequent uprisings.
The majority of these uprisings happened because of the dire poverty most Irish people lived in while the Irish nobility and its merchant class kept fat cattle and the best wheat for export (to England and Europe) so that they could fill their own coffers at the expense of their countrymen.
The Irish, like the Americans, perpetuate the lies and deceptions that were used against the English right up to the present day, and they (the Irish) seek apologies from the English for what they did to themselves.
2. The Welsh have been closely united with England for hundreds of years, and Plaid Cymru is still a minority movement. There is no real hatred between the two countries, just a healthy rivalry. True, the Welsh have a keen desire to preserve their national language and heritage - looked down upon by English speakers until recently, but that is a good thing, and it does not demand that they cut their ties. Incidentally, English is spoken by more Welsh people than Welsh.
3. The Scots, too, are unlikely to become independent from England, and if they do, it will be by agreement between friends. Again, hatred of England is historical and largely unfounded - unless, like the Irish, you go back several centuries into history to justify your claim of unjust treatment.
The United Kingdom is exactly that - a union of two kingdoms, a principality and a province, and each has its place in that union just like any state in the USA. (The province is that part of Ireland which chose to remain British, after the establishment of the Irish Free State. It seems there are many Irish republicans who cannot countenance a British presence to the north, just like the American republic found it necessary to go to war against Canada after their independence.)
Most of the English/British possessions (save those where we had a peacekeeping role) sought independence by peaceful means and obtained it. The vast majority of those countries remained in the Commonwealth where they each have an equal voice on matters they deal with. A number still have the Queen as their head of state, and Britain is held in deep affection.
So, to sum up, England has committed many atrocities through its high handed arrogance and belief in its superiority over native peoples - that seems to be part of the Germanic DNA. I believe Britain is also guilty of genocide, where it successfully wiped out a whole race. We have ruled some places very badly.
But we have also ruled well, and the evidence of that is to be seen all round the globe. Some people, however, justify their own bad history by blaming it on us. The Irish were the authors of most of their own problems. They chose to become a third rate country rather than be part of the United Kingdom (an equal pert, remember). Zimbabwe shook off British shackles in order to murder white Zimbabweans and steal their property ... and that country is in financial ruins despite all its natural wealth. The 13 colonies were seized by a motley crew of smugglers, pirates, profiteers and other malcontents who sought to further their own interests rather than their compatriots and condemned them to years of war, to higher taxes and a national debt and called it "Liberty". There was no more freedom - in fact there was now less freedom - than they had under the British.