...so basicly im running 100psi which is very low but ovb enough to run, as regards it picking up at around 45000rpm does this add up would it have jigher compression at higher revs?
I am afraid it does sound something like piston rings, they dont last long if they have been allowed to run on a low oil level, which sounds much more plausible to me than just wear and tear... 91000 miles is not alot for a well maintained MINI Cooper... I service one for someone that has twice that and its fine!
If you are lucky, it will only be the rings that are worn, but in my experience, the pistons follow soon after the rings as the worn rings rattle around in the ring groove in the piston and wear them out, needing the pistons to be replaced at the same time. From memory, a set of rings are around £100, but I have never had to buy them on their own as I always needed pistons, and they come with the pistons.
With regards to the engine picking up at higher revs, its all to do with speed against torque, the higher the piston speed, the less low compression will have an effect, within reason, as there are more 'bangs' (power strokes) a second to 'share' the load and therefore the drop in compression is less noticable to you... but it will still be down on power.
To fore arm you with prices for when/if BMW try to take your trousers down!
I replace pistons and refresh engines for our race cars (everything in them has to be standard) and the pistons are the expensive bit, as, I said, they come as a complete set, about £650ish (Ihave seen sets on the internet for £400 but we cant use them, so I dont know what they are like), but then you have to buy bearings and all the gaskets/seals involved in the job, roughly, you are talking about £1100 on the engine alone, so long as the block is not damaged, which is not likely unless they ran it dry for a while.
You can get a second hand engine from a breaker, or an 'e-bay special' for around the £500 mark, I have seen them sell for less, but never risked it.
Obviously doing your current engine is the more expensive route, but at least, if done right, you will at least know you have a good engine that will be good for at least another 100,000 miles... with the 'e-bay special' you really dont know what you are getting... despite the breakers claims that the engine has only done 20k, there must be loads of MINIs scrapped with 20k on the dials!
In either case, you are going to have to pay someone to remove then refit the engine. It officially should take 6.1-6.35 hours, depending on extras fitted (our record is just under 2 hours, using 2 men). At dealership prices you are staring at the wrong end of a £600+ cash transaction for that bit alone, then there is the initial diagnostic and anything else that crops up, clutch replacement, likely at 91000 miles, etc. Also, dealers are less likely to go down the route of supplying you with a second hand engine, opting for a BMW recon. unit... a steal at around £2500!
My recommendation is the independants route where you should find someone to do a replacement engine job for around the £800 to £1400 mark, including supplying an engine.
My Guesstimation (using past experience of the race cars)-
£1650 all in for engine refresh
£950 for a replacement second hand unit
Hope this helps