You're challenge with a Pod after the death of Henry the 8th for Britain to return to Catholicism and stay that way till the present day.
The numbers rise of course if you include the number of people killed in Ireland during the various Irish rebellions of the late 16th Century but to be fair the focus was largely on rebellion against the crown and the religious motivation was a secondary consideration.
Wolfpaw, such a POD is indeed the easiest way to keep a largely Catholic Britain, especially if Mary herself lives a bit longer - or if Edward dies sooner.
However I took take umbrage at the reference to "oppressive policies";
recent scholarship shows that that is Elizabethan propaganda.
Without defending the execution of the Protestants, it is worth pointing out that unlike the Elizabethan persecutions - which were really oppressive, and designed to be so, the vast majority of the executions were in fact locally based; Mary's government actually tried to prevent them.
Elizabeth had more Catholics executed in London alone than the Roman Inquisition in the same period. Mary's pale into insignificance in comparison.
What made England Protestant was Elizabeth surviving 45 years so that by by the end of her reign, close enough to 2 generations had received sustained propaganda along the lines of "Elizabeth=England=Protestant".
Read Eamonn Duffy's "Stripping the altars" among other texts.
There is however a debate over the meaning of catholicism in that the Church of England regards itself as following the apolistic succession and many Anglicans regards themselves as following the cathloic tradition with the term Roman Catholicism being applied to people who regard the Pope as head of the Church. The title Defender of the Faith was conferred on Henry for an essay attacking Martin Luther
A possible scenario would be for the Pope to have granted Henry's annulment and Britain may well have stayed in the fold. Would the industrial revolution have taken place in Britain given the Churches attitude towards commerce? Would an entrepenurial merchant class have arisen in Britain or would the industrial revolution have taken place in Northern Europe?
[SIZE=2 said:. The situation was even worse in Scotland where the presbytarian church was Calvinist in nature and was even further removed from the Catholic tradition that still had echos within the English church in fact the prejudice in Scotland had far more to do with religious belief than in the rest of Britain. The overriding problem of course was that Catholics could never answer in the view of Protestants the eternal question whose subject were they? - where they loyal to the British Crown or to a foreign Prince (the Pope) and in that the endemic xenophobia (for which the English were known for by the 16th century) played a big part.[/SIZE]
That suspicion of Roman Catholicism lasted far longer than the laws that excluded them from roles in Government and again came to the front during the mass Irish immigration into the UK mainland during the 19th Century. Also British views of Catholicism have in the last century have largely been informed by the Irish experience (i am not so much talking about terrorism here) where until the last decades or so the church had become so entwinned with government and education within the Republic a complete contrast to the role of the church in the UK and in other nominally catholic countries like France or Spain.
I am not convinced by arguements that had Mary I produced a child and lived longer or had the Pope granted Henry's annulment that the reformation wouldn't have reached British shores (Scotland's reformation happened without Royal sanction for example). Britain was remote from Rome and there was already a significant strand of anti clericalism and resentment.
If it doesn't happen then it is such a significant shift that you've wiped out a great deal - no growth in Parliamentary power and influence ( The Tudors relied heavily on Parliament to legalise their religious changes which in turn increased the role and influence of the commons), reduction in the wealth and power of the non-aristocratic tudor gentry (the men who dominated the shires for the crown and who grew rich from confiscated church land - almost all modern english aristocrats descend from these families along with most of England's most influential politicians of the intervening centuries), no union of the crowns at least in the immediate future and therefore more wars between the two countries, completely different issue with regard England's control of Ireland, no war with Spain to dominate the later half of the 16th century, strong possibility of England's colonial expansion stalling or being delayed, no export of the English view of Parliamentary democracy meaning significant changes in styles of government around the world - I could go on.