The last I knew was that there were only a couple of exams difference between the two. Frankly if you have plans to go professional then you might as well do them all and get them out of the way. That said, they have a time limit once completed so if you expect to take a long time to change career then you need to consider the cost of losing validity. If you haven't already got the IR(R) then do that before you do anything else. You can operate IFR throughout most of the UK zones and CTA's without needing the full IR.
To be honest, with the base of Class A airspace having been raised in most regions of the UK other than around London you're hard pushed to actually utilise a full IR anyway. I'm based at Hawarden and going South, West and North most airspace is available to me up to FL100. Going South East to London I can still maintain a safe MSA in IMC but would need to use my full IR to reach the minimum transition level in the CTA which is FL100, so if you're flying something that would struggle to get up there at less than 500'/min you'll not be welcomed and will probably be vectored out of controlled airspace ASAP. I believe there is a well known syndrome called 'Being Dumped at Detling' which describes this issue.
The other thing to consider is what capability you have in your aircraft. Flying IFR in the UK is impossible in winter without de-icing and near impossible in summer due to convective cloud between five and ten thousand feet. You need at least a primitive two channel autopilot with heading control if operating single crew otherwise you'll find yourself looking at a violation if you bust your assigned altitude by 200' or more.
For now, use your IR(R) to its full extent on the days when the weather permits smooth ice free cloud flying and then see where you want to go career/capability wise after that.