Any improvement in economy won't pay for the cost, and power gains will be more of a placebo, IMO.
Your best starting point is an FRP exhaust system, this will release horsepower and allow the engine to run more efficiently. Won't have a huge impact on your fuel economy, but I would be suprised if it got any worse as a consequence.
Your problem will be a lack of used parts locally, and a general unwillingness of UK eBay sellers to post abroad, probably worth a try though. Also keep an eye on the For Sale section on zsoc.com.
Without wanting to sound defeatest however, you might be best off just making sure your car is currently running at 100%, while saving for something more powerful.
150-160HP is achievable in a Puma, but it isn't cheap. You need FRP exhaust, cams, flowed head, either FRP or custom map, and ideally also FRP inlet manifold. Could easily run to £2000.