Mountain bike rain coat / jacket (packable, not for winter)
Recently introduced by 'Altura' (the poor British man's 'Endura') is the 'Pocket Rocket':



This jacket is distributed by Zyro and widely available in British bike shops. Visit a stockist and try for size wearing what you expect to be wearing underneath, i.e. more than a T-Shirt. Remember that when buying from the LBS you may pay RRP rather than 'online price', however, you won't pay postage, have to wait or be bereft of a coat if a seam goes and you need to take it back. (Altura seams are pretty good b.t.w.)

Available in black, yellow or red versions of hi-viz, the reflective parts of this jacket will dazzle on-coming motorists. A neat feature are the thumb loops that enable you to keep the sleeves tucked under your gloves.

IMHO a lot of 'breathable' does not work, so there is no point bothering with it. There is some rating of breathability for this jacket, but you can expect some 'boil in the bag' aspect. Waterproofing is claimed 100%, just what you asked for. 

A pack-to-nothing jacket is truly useful to just have. It is also nice to have a relatively plain jacket without any fancy colours or extra seams. This jacket keeps it simple. The pack size is exactly what you want, i.e. very small:



You can dangle it on the handlebars if you are in casual clothes and without a bag. If you get this jacket, expect people you know to want to permanently borrow it. Also expect to see other people on bikes with a jacket remarkably similar to yours.