It can often be frustrating when a busy junction has lights phased for quiet times. Using any alternative route from the designated and expected ones increases risk, so should be handled carefully.
If you are in a lane with directions in it, such as an arrow, failing to follow that route could result in a ticket for failing to comply with a traffic sign, or at least a lengthy discussion with a policeman. So if choosing to go ahead and then do a U-turn, as long as you are using the ahead lane, there are no issues there. Anyone arriving in an ahead lane, then realising they need to turn right would likely do a U shortly after the junction.
Doing a U-turn is only illegal if there are "No U-turn signs". Such a manoeuvre must be carefully considered though, as, if it results in a collision, you may be prosecuted for careless driving.
Other traffic that notice what you are doing may get annoyed with you. We are a nation that likes orderly queues, but easily annoyed by queue-jumpers.
If your actions encourage others to do the same, it will quickly develop into silliness when busy. I have seen a similar situation where the crossroads is blocked by a queue, all waiting to do their U.
If the crossroads operates adequately at quiet times, consider writing to your local traffic department at the council, outlining the problem, and asking if the lights can be set to phase differently at rush times. Be polite and clear, keep emotions out of the letter/email. It is likely that the traffic department have only ever visited at quiet times.
Is there an alternative route you could take?