Having issues with your speed isn't always great and you have to understand VM will always try to provide those speeds for you but just like any other ISP it is an "UP TO" speed and not guaranteed, a guaranteed (Business / Enterprise line) are very very expensive in comparison to the speeds and could be in the thousands for a dedicated line.
Now I know this is not an excuse but leaving VM to go to Sky / BT will not change it and you will most likely experience the same issue with them also and of course BT lines are not as good as VM and can be even worse depending on your area. (I've experience 2mbps with BT)
Lastly where are you getting these speeds from? Can you do the speed test on http://ovh.net only speedtest i'd really take for granted, and with concern for the price i'll tell you after the fancy offers vanish and your stuck in a contract with BT I was paying more than £40 for there pathetic fake fibre connection.
"I have not failed. I've just found 10,000 ways that won't work." - How everyone in a technical field should think.
If at every single upgrade they fall short by 30 megs of speed it means that it is being deliberately throttled and specifically managed to be this way. Please don't insult peoples intelligence. I'm a 20 year network engineer and know full well how this works.
If you promise and sell up top 100megs but only deliver 60megs then you should adjust your pricing accordingly and only charge for what you are actually delivering and not some fantasy 'up to 'figure you just pulled out of a hat.
Looks like I'm not the only one with this issue.. almost every single thread in this forum is about virgins non delivery of advertised and promised speeds. This is appalling.. false advertising and dishonest practices greedily delivering less and less but charging more and more.