It could be, it might be useful but it isn't necessary to have a CS degree in order to be a programmer.
Are you planning to take a year off work to do this or do it over a couple of years part time/distance learning?
I guess one useful aspect of it could be just the signalling aspect - getting higher ranked university on your CV + having access to a careers service. (Though Manchester is of course already a good university.)
The benefit perhaps of doing one full time is that you might well have the opportunity to carry out your dissertation with an external advisor in industry, possibly paid too - this obviously can lead to full time employment with that company upon graduation. Some universities, by nature of their brand/ranking, will have plenty of these opportunities.
For example:
https://www.imperial.ac.uk/study/pg/computing/computing-science/
https://www.ucl.ac.uk/prospective-students/graduate/taught-degrees/computer-science-msc
Alternatively if you're going to go for the part time option then this might be an option (given some experience) albeit it isn't a conversion course per say and does allow for a potential security specialism. They don't seem to have a CS degree as a hard requirement but do seem to require some experience, probs best to contact them:
http://www.cs.ox.ac.uk/softeng/
Alternative to a conversion degree as a means of getting some experience/employment in a potentially shorter time period - you could take a look at the nanodegrees from Udacity (or indeed other MOOC providers)
https://eu.udacity.com/nanodegree
If one of those lead to a job then perhaps the above part time option for an MSc might become more feasible too.
Incidentally, given your focus on bioinfomatics in your undergrad - have you thought about data science careers? Might want to look at an MSc in Stats or similar... Sheffield has a distance learning option, Imperial is apparently going to be launching one in ML too via coursera.
Just a thought as data scientist in say a big drugs company could potentially land you double what the typical developer outside of finance tends to earn. Your undergrad degree is then potentially still relevant, especially the 3rd year modules and you're also (potentially) partially involved in development too.
