The police departments can access the call logs and websites data but GCHQ have more privileges such as hacking into suspects computer based on reasons only. Previously, a judicial grant was required to conduct such actions. But now, approval is required from the PM and specially appointed judge for warrants, additionally, they can bypass these authorizations if there is 'urgency.
http://www.vpnlogics.com/uk-investig...wers-act-2016/' I think everything would be pretty much urgent from now on. The only thing we can do is to protect our online activities as much as possible.