Hi Kate,
I assume you are wanting to use electric/battery power as part of your zero-carbon commitment? In my experience, it really depends how tough the job is as to whether or not a cordless strimmer will do the job. My cordless tools are Makita (only because I already had some batteries from cordless drills etc.) and, although they are great for the lighter jobs, they are not quite up to the tough stuff. Our cordless strimmer is quite capable of doing just that, strimming grass, but struggles with tussocky stuff. There are heavier models described as cordless bush cutters but, by the time you add batteries, they come in at well over £400.
It may be worth phoning any tool-hire companies in your area to see if they can offer anything but not many are offering cordless garden tools yet. Or try and borrow something in the short term to prevent you having to make a rushed purchase decision.
If we were closer I’d offer for you to try our ‘strimmer’ but getting over to Llanidloes would create a larger carbon footprint than hiring a petrol bushcutter for an afternoon 🙂