Further to the answers already given:
"Right of Way" is not a principle recognised by any UK traffic law or regulation (it is more appropriate to rambling over the countryside). This phrase is often used when "priority" is meant. In the UK, priority is always indicated by specific signs or road markings. An arrow directing lanes to merge does not offer any priority; each vehicle is expected to merge in turn much like the teeth of a zip.
The fundamental rule applicable to motorists in the UK (as exemplified by the
Highway Code) is "Give Way" and failure to do so "falls below the standard expected of a reasonable and competent driver" - aka the offence of "driving without due care and attention, or without reasonable consideration for other persons using the road".
Thus if you attempt to block a vehicle ahead of you from merging you commit an offence. Likewise, a driver attempting to push ahead and force a way in commits an offence. If there is a danger of injury or serious damage then the offence is escalated to that of Dangerous Driving.
If a collision does occur the difficulty in assigning blame may be in trying to differentiate between the two actions listed above but that is the job of the insurance providers.