You also have to consider the other benefits of this proposed section of LGV.nearly half the cost of this plan is to allow the line to serve Heathrow via a big expensive tunnel under West London. It's about £8billion without it, though isn't really the best scheme, though the fundamental concept is excellent - a line to bypass the overcrowded southern ends of lines to start with, reducing times from further north.
With regards to the Maglev - is it going to go into city centres, as that would involve costly tunnelling? or is it that you'd have to change at parkway stations? In which case, you have to compare with a London-Birmingham LGV going between the M25 and the M42, which will further reduce the price of the LGV.
The Greengauge proposal for example has a link from it's Heathrow loop connecting onto the GWML just West of the airport. With a connection from the HSR onto the six track approach to Paddington that's a bit of extra capacity for Great Western services chucked in.
The Maglev proposals seem to focus on outer urban stations, in which case you have to add on the time it takes from city centres to these stations to the overall journey. While LGV's also rely on outer urban stations aswell they still have city centre to city centre services aswell.
In all reality we are probably only going to get a full north HSR in sections so get this done first and then work on extending it to Manchester and so on.