500mbps but getting 8mbps

After having connectivity issues last week, on Sunday I tested my internet speed. After 20 tests it ranged from 32mbps to 8mbps, should be 500mbps. Phoned today and the man I spoke to from virgin media, didn't know what mbps was. I had 3 months last year of virtually no internet, to make up for this they set my monthly cost to £41.00 for the next year. Since April they had a price increase to £44.60 and they have been charging me that. I can get more than 8mbps from connecting to my neighbour's, why do virgin believe this is acceptable. I have tried to stick with vm since they first came on the scene, but enough is enough. Basically £45 per month down the drain.