Thank you everyone for the helpful replies. I have finally found a local family mechanic whom I have some family ties to and I trust, so I'm going to ask him to have a look at the exhaust/brakes/springs on Friday. I'm just reluctant to spend big money getting a car through an MOT that is literally worth nothing more than scrap, even if it is a nice car(no-one wants these gas guzzlers any more!). I spent ~£350 replacing the track rods etc for the last MOT, which stung a bit. I've also replaced the rest of the exhaust a couple of years ago, so annoyed at having to do more exhaust work.
Obviously the tyres and brakes are wear and tear (although front pads replaced last year too), but it's just unfortunate to need to replace all of those at the same time as paying for other MOT bits AND my road tax (which is extortionate, seems to go up every year!). The car is low mileage and is tidy enough inside and out. The rough running and lack of power I put down to spark plug/coil pack issues (which also needs attention) is probably more down to a gaping hole in the exhaust though, right?
dbdb - I think the MOT testing station may certainly have been being over zealous. I phoned on Thursday to make a booking for Saturday and they booked me in for 11am. When I turned up they told me there was no record of me and that they would "try to fit it in", but acted like I had done wrong and was a big inconvenience to them. I knew there and then it was going to fail. Certainly one of the tyres was worn, but going by the wear bars, the other two had life in them yet.
Taxi Driver - The car doesn't pull when braking. Off the top of my head I think the readouts were 28% and 4%, although I could be slightly out.
nitro - The car has been in the family since it was one-year-old, I know its history and it hasn't covered many miles. Looking around at cars in my price range (up to about £1,500) I would have to downgrade, go older and more mileage. After having a look around I'm leaning much more towards repairing the Omega. Although it all depends on my quote on Friday. As ronnyd says, I could downgrade and still end up with the same problems in six months when the replacement car needs an MOT.
Just to further complicate matters - I'm emigrating in May, which further muddies the waters as to whether it's worth spending money on it or not.