'Reformed' could be used in a purely historical sense. In the 16th Century, the Protestant Reformation led to the emergence of three great Protestant churches: the Lutherans, the Anglicans, and the Reformed. (On another reading of history, there were maybe just two big strands, with Anglicanism being a subset of Reformed. I think this is less useful, descriptively and analytically, just because of the huge differences between Anglicanism and other 'Reformed' churches). Obviously, there are now churches which would claim descent from the Reformed wing of the Reformation. It is a fact that many of them, particularly on the Continent - although one could also think of the URC, now tend towards a theology which is either a significant modification or a near-total repudiation of the theology of the 16th Century Reformed. Still, they have an obvious claim to the title. Perhaps we should speak of church today which have 'Reformed heritage', thus acknowledging the link without making any theological claims.
'Reformed' could also be used in a theological sense, in at least two ways. We could identify a 'thick' Reformed theology and a 'thin' one. On the thick understanding of 'Reformed', we would be talking essentially about the Christian Institutes of John Calvin. Reformed theology is covenantal and paedobaptist, characterised by a particular view of the relation between Word and Spirit, cessationist with regard to miraculous gifts, presbyterian in church polity, placing a strong emphasis on the sovereignty of God in revelation and salvation. I think you could make a good argument for this being the most comprehensive definition of 'Reformed', and the most useful, for reasons to be explained. We could perhaps talk about churches which espouse this theology today as having 'Reformed doctrine'.
The 'thin' understanding of what it means to be theologically Reformed tends to take a subset of the beliefs above, which are taken to be the 'heart' of the whole system, and makes these beliefs the meaning of 'Reformed'. This tends, in practice, to mean being Calvinists in terms of how people are saved, espousing a high view of the sovereignty of God. There are many churches that would take this line, and we could perhaps talk about them having 'Reformed soteriology'.
So, who is Reformed?
On the first understanding, the URC is and newfrontiers isn't. The URC stands in a direct line of succession from historically Reformed churches, whereas newfrontiers does not.
On the second understanding, the URC is not, nor is newfrontiers, nor is any baptist church. In the UK, you could go to the Free Church of Scotland, or perhaps the EPCEW. You'd struggle to find any other churches that are doctrinally Reformed in this maximal sense.
On the third understanding, newfrontiers is Reformed, as are all FIEC congregations I've ever been to, as are many evangelical Anglican churches.
My own judgement would be that we should use 'Reformed' in the second sense, primarily. I think it is useful to give a word the maximum definition it will take, for purposes of clarity and analysis. I also think it is clear that the historically Reformed would want us to think of theology and not merely heritage as the defining mark of their movement.
So I think only presbyterians are Reformed. We could perhaps use qualifying language to show the relationship that other churches stand in to this tradition - whether historic or theological. Personally, I am an anabaptist, holding (lightly modified, but still very clearly) Reformed soteriology and a (more heavily modified) Reformed understanding of Word and Spirit. Perhaps I could say I am an anabaptist with a theological debt to Reformed theology. But I'm not 'properly' Reformed, and I'm okay with that.