Using the data that kevinlarkin has at his disposal (which is likely to be changed once the electorate data dump happens in January) here is my first stab at Northern Ireland
Constituency Descriptions and notional status Belfast, Titanic 7.58% of Antrim East 93.21% of Belfast North 6.08% of Belfast West Ultra Marginal Sinn Fein
The Titanic Quarter is the other side of the river in East Belfast, so can't be used as a descriptor for this. Without seeing the detail, some of those changes seem unnecessary. This has been discussed elsewhere but the west is mostly fine. All that needs to happen is West Tyrone gaining a ward from Foyle. - East Antrim needs to gain 3 wards from North Antrim. Glenwhirry, Slemish and Glenravel seems the best option, but the other is extending north to Kinbane. - Belfast North gains Hightown ward from South Antrim. - Belfast South and West both need no changes on 2015 figures, though West is slightly undersized on 2020 figures. It gains Derryaghy from Lagan Valley. - Lagan Valley is ok after that loss, but gains Aghagallon ward from Upper Bann to allow changes there. - Belfast East gains 3 wards from North Down. - North Down gains the Ards Peninsula from Strangford. - Strangford gains 6 wards around Downpatrick from South Down. - South Down gains 5 wards around Banbridge and Loughbrickland from Upper Bann. - Upper Bann gains Loughgall and Tandragee wards from Newry & Armagh.
The electorate figures published on the EONI site don't suggest much has changed since 2015, though, confusingly, they use the old wards, making it difficult to calculate.
There was certainly one authority where, in the rush to ensure the inclusion of all the new registrations for the GE, they duplicated many people on the roll. The affect was that the roll was much bigger than expected and the turnout was apparently much lower than in neighbouring boroughs. I believe that authority cleansed the register in the new year.
For information-I posted elsewhere that the March 2nd data will be available on January 5th 2021.
The GE total electorate in England was 39,897,593 so 1.1% higher.
That is interesting because on the basis of Dec 2019 electorates Wales (excl Anglesey) has only a marginal claim to a 32nd seat. If the Welsh electorate shows a similar decline of 1% or so between December and March, then Wales may lose that seat and end up with 31 + Anglesey. If so, the extra seat would presumably go to either England or Scotland.