I have almost the same problem. I upgraded to 0.5Gbps, and although my upload speeds increased to around 35Mbps, the download speeds are now less than the 200Mbps service I had previously. It's regardless of the device, being the same across various laptops & mobiles, wired or wireless.
I should have known better, Virgin being Virgin, where customer service is treated as an inconvenience to Virgin.
I'll have to drop back down to 200Mbps in order to increase my download speeds from a 500Mbps service!
Virgin really don't have serious competition with the speeds they offer, perhaps that's why they treat customers like idiots?
Thank you so much for your first post to the forums and it's great to have you on board!
I am so sorry to hear this is happening to the customers services. I would love to have a look into this for them, but I would need to be able to find their account online. I'm going to send you a PM so we can try and get this sorted. Please keep an eye out for the purple envelope in the top right corner of the screen.