Even though I am not originally British, I am currently living in the UK. I would say that a Master´s degree, whether it is integrated or as a further postgraduate study, can make a difference.
I have two main points to support my reasoning.
The first one is my personal experience, I have obtained a place in a top engineering organization thanks to my Master´s (among other things), the Master´s was one of the requirements to get into this particular graduate scheme and I will be encouraged and supported to obtain my registration with an Engineering Institution. No matter how good my CV had been, I wouldn´t have been able to get into this grad scheme without a master. I also tried to apply for grad schemes with my Spanish BEng and 6 months of experience but I was completely unsuccessful. Getting into a good uni also helped me a lot to get the inside track of how to look for a job.
I know that with your extensive experience you won´t be applying for graduate schemes. But I think that a Master´s is a requirement to register as a Chartered Engineer in most of the Engineering Institutions. Getting Chartered is becoming the standard that most enterprises are looking for.
In general, I would say that a Master´s can definitely make a difference of more than 25,000 pounds (15,000+10,000) in your career if you take into account the doors it can open. I would also like to say that jobs won´t come to you after you get your Master´s, but it can definitely make a difference.
Finally, in case that you choose to go for a Master´s, it would be a good idea to pick up a good uni, by good I mean any of these unis: http://www.russellgroup.ac.uk/
The UK is still pretty classicist and having a good degree from a good uni is quite valuable.