jayemm
Life jackets are £50-£90. Nearly all those on general sale for the GA market are identical internally; are contract manufactured by Remploy and the spec varies a little; e.g. the more expensive ones such as SEMS (also sold by Transair at Transair prices) have "less sweaty" fabric.
Life rafts start at about £1100. You can get them for a bit less but those are without a canopy and thus no good for cold driving rain; in heavy seas only marginally better than swimming. A canopy is essential. I've got one but if I was getting one again I would get one from RFD; they sell off their website and are much better value that the American ones sold for the "aviation" market. Life rafts are supposed to be serviced regularly (not a UK legal requirement for private flights) and if you buy the "wrong" one you can find that the service is expensive because nobody stocks the replacement bottles, etc - I got burned on this myself.
www.rfd.co.uk is worth a look. The drawback of the RFD ones is size/weight.
For locators, the old ~200MHz ones which are still widely sold are a waste of time because the satellite location has poor accuracy and too many false alarms to be useful. You need the 403MHz one and they start at perhaps £500, slightly more if a GPS is inside. It can be argued that an integral GPS is not necessary when flying in the civilised world because SAR services are common, but elsewhere you may be relying entirely on commercial shipping (most of which nowadays navigates using GPS and they can sail directly to a given GPS location) and a built-in GPS is essential.
www.sartech.co.uk is one place to look.
Dry suits I have no idea about.
Personally I always fly with a bag containing a handheld GPS and a handheld transceiver. These would be dead handy if you end up floating somewhere, couldn't do the mayday call (or nobody heard it - easily done around France on a Sunday) and the EPIRB is duff. You can easily call up an airliner at 30,000ft, even 100 miles away - I gather they monitor 121.50 en-route.
I have never ditched but the above appears to be the condensed wisdom from people I have spoken to when researching the subject.