A high-speed VPS has a high port speed. It allows lowering latencies, provides fast read, write, and I/O speeds. A total port speed of 40Gbps is considered fast, for example. Of course, to achieve such high port speed the server must use very fast processors. A fast processor can be the Intel E5-2697. But that is not the only one available on the market. However, if your website will demand many resources, you should consider a provider with multiple CPU cores.
Moreover, the speed of the hard drives and (to a lesser extent) the RAM plays a role. One of the most important factors to consider is the hard disk I/O. If the disk is slow, the UK VPS server or node will be slow too. The use of SSD-drives will contribute significantly to making the server much faster. This is why you should choose a VPS provider that has at least RAID10.
As an example, a 10-fold increase in speed can be achieved in comparison to common SATA hard drives by using solid-state technology. Hence, do not even consider VPS providers with 1 or 2 SATA disks if speed is your concern.
More advanced technologies like software-defined storage (SDS) have a positive impact on speed too. As for RAM, look for at least 4096 MB. If possible, make sure the provider has Burst RAM in place.