Yes, good point about about a viewfinder - either an add on one for the GX1 or the built in one of the G3 for example would help a lot when using long lenses for birds etc.
The G3 is good value at the moment at around £300 after cashback with 14-42 kit lens. The G3 is a dslr styled camera (kind of like a mini dslr) whereas the GX1 is shaped more like a rangefinder or compact.
(You would need the Panasonic DMW LVF 2 add on viewfinder for the GX1, which costs a hefty £200).
The micro four thirds cameras with built in viewfinders are: Olympus: OMD, Panasonic: G3, G5, GH2, GH3 and the Sony Nex cameras with built in viewfinders are the Nex 6 and Nex 7. Of these, the G3 is by far the cheapest as it's been replaced by the G5.
Another slightly different option might be the Sony A37 dslr (technically an SLT) at just over £300 with 18-55 kit lens after cashback. Yes it's a dslr/slt but it's not actually
that much bigger than the G3:
http://camerasize.com/compare/#326,185 and could be paired with the Tamron SP 70-300 for birds etc:
http://www.dyxum.com/lenses/detail.asp?IDLens=595
It has fast AF and a nicer evf than the G3 and you can also use any of the older Minolta AF lenses many of which can be bought used at reasonable prices. Tamron and Sony also make a budget 55-220 lens (£89/£120ish) which is excellent for the price. Dslr/Slt Lenses will be significantly bigger and heavier than micro four thirds lenses though - although some of the plastic Minolta lenses are light and suprisingly good. I got a 35-70 Minolta lens on ebay for £10 which is just as good as the Sony kit lens (if not better) and just as light, although much noisier!