I can't really speak for the OP, but I'm kind of guessing that what he's looking for is a party that subscribes to most or all of the things that the mainstream left subscribes to, AND is English nationalist. Admittedly, if he's willing to accept anyone with a collectivist vision as being left-wing, that makes his challenge a lot easier.
More or less. Many parties can be defined as collectivist, what is more important to one being seen as left wing is that it defines membership of the collective on a broader basis, whereas some ethnically nationalist parties like the FN or the BNP will be in favour of economic intervention only so far as it applies to people like them,for example advocating defunding state education of foreign children, which is why I made a caveat against them in my OP.
I'll allow parties that are socially conservative in some respects but whose ideology is primarily based on what are understood to be left wing principles, such as Communists, but I doubt anyone is going to engineer a scenario where an English Nationalist Maoist party is getting over 10% of the vote in modern Britain.
Um. Both UKIP and the BNP are already left wing nationalist groups in favour of social intervention already surely? Both are well to the left of the Conservatives, Liberal Democrats and even the Blairite wing of the Labour party on matters economic. Read their manifestos.
UKIP are a party that is largely composed of those inclined toward Thatcherism, appealing to those who are economically left wing but socially conservative. As a result there 2015 manifesto was probably more economically to the left than there membership, but I still wouldn't call them economically interventionist. They pledged to scrap inheritance tax, bring down the top rate of tax to 40%, end 'green taxes' to cut fuel bills, lower the benefit cap, and eliminate the deficit on the same schedule the Tories pledged too.
The 2010 manifesto included things like school vouchers and shrinking the state to the size it was in 1997. Even there main 2015 pledge when it came to redistribution, raising the tax threshold to £13,500, was probably more economically liberal than interventionist. Granted, there were a few somewhat left wing ideas in there, but not nearly as many as right wing ones, and some of it was collectivist only when it meant taking away from another group, for example, there pledge to cut most of the foreign aid budget and spend it on the NHS.
The BNP is arguably more interventionist, but mainly in that same sense, increasing welfare provision for those who are part of their more narrowly defined collective and taking away from those who aren't. In any case, they come under 'outright ethnically nationalist' with there pledges to deport immigrant and ban building of mosques, which is why I said no Front National equivalents.
So what I am looking for is a party that is generally accepted as left wing by most commentators and the mainstream left, whilst also being civic nationalists.