The semantics of the word "conservative" pairs it with "progressive" or "radical" as opposites. It does not pair with "labour". It is a historical accident that the United Kingdom has a "Conservative and Unionist Party" and, as its main opponent, a "Labour and Unionist Party" - though it doesn't include the Unionist bit in its name.
Time to change. England - London and maybe a few other cities excepted - will be dominated for the foreseeable future by an ageing electorate which doesn't think much beyond house prices, free bus passes and royal babies. The Daily Mail will shepherd them into the polling booths to Vote Conservative, not that they will need much shepherding.
To oppose the Conservatives, you need just one broad-based Progressive Party. But it should be a party committed to a Federal Britain. In that way, it could accommodate - until such time as they had support for full independence - the Scottish National Party and Plaid Cymru, both of which espouse Progressive policies. For both a Federal Parliament and devolved parliaments, the SNP and Plaid would not be opposed in the constituencies where they had candidates. There is no reason in principle why those two parties should not be paralleled by an English National Party, also running candidates under a Progressive umbrella. But this might not be necessary. If a Progressive Party was created out of the ruins of the Labour Party and the Liberal Democrats, then it might be better if it ran candidates for both devolved and federal parliaments in its own name. But a Progressive Party would not oppose the candidates of the SNP and Plaid in Scotland and Wales.
Northern Ireland cannot be accommodated in this scheme. There are no Progressives there and the statelet should be encouraged to federate with the Republic of Ireland, thus re-unifying the island. If it's desirable for Cyprus, it's desirable for Ireland.
A Federal Parliament on a US or Australian or Canadian model would concern itself with foreign affairs, defence, borders, currency and banking and seek to achieve some consistency in major policies across the devolved administrations. The Federal Parliament would be small and might not be based in London. Alternatively, the English Parliament might not be based in London.
Even without federalism, a Progressive Party of England could decide not to field candidates in Wales and Scotland simply as a means of trying to ensure the largest possible Progressive bloc in Parliament.
You can work out the details and problems in your own head. The important starting point, however, is the recognition that it is not just the UK electoral system which is unacceptable to anyone with the slightest democratic inclination; it is the field of failed and failing English political parties with which the voter is confronted.