I've just found this site for reporting traffic incidents:
I started logging something bike-related I saw on Friday, to test it, but must have struck a particular combination on my keyboard and lost all the information I had input. Not sure I can be bothered doing it again...
Anyway, back to your question. I can understand both sides re the use of Twitter. We want something quick and easy for those 'not important enough to call 999 or 101 but might be useful for the police to know' pieces of information. The police want to use Twitter as a means of outward communication with maybe a bit of lighthearted interaction with other users. Practicably, we can't expect them to monitor their Twitter feed throughout the day, on the off-chance that a member of the public has logged something important. They don't have dedicated social media staff who spend all day on Facebook and Twitter. And they would have to log the information in their own systems which would be administratively time-consuming. Plus the information may not have specific details that they need.
That reporting website is asking for information that the police know to be relevant (eg road conditions, amount of daylight etc - although that's more relevant to a traffic incident rather than suspicious bike riders) plus the burden is on the public to enter the information, not a 101 call handler whose time would be better spent dealing with calls that need a response rather than typing up information someone is communicating via telephone.
In the past I've e-mailed PerryVale.firstname.lastname@example.org with that type of information, but thinking about it now I'm not sure how integrated they are with other police departments, eg would the information in the incident reporting website be relayed to just the traffic police and not the local neighbourhood team?
I see there was a Twitter post asking for sightings to be reported, via 101. Don't think I would bother ringing with my 'just in case it's useful' information from Friday evening to maybe I will give that website another go.