You should check out eBay, there's a surprising a large amount of chart plotters available, some with all the goodies and some without. Be sure to check which ones are in the UK as there's quite a few on eBay Uk that are located in the USA so shipping will be high and no UK map.
That Lowrance HDS7 above would be a great price if it's the Gen 2 version and has all the bits you need, perhaps you should send him a PM.
You will also see a lot of Raymarine's C's on there as well. The prices for them can be excellent as well as there usually a high amount/volume of them for sale.
The biggest consideration is what you need for the plotter and if the one your interested in has all of them. Like a mounting bracket, Cover, cable, antenna and recent UK chip....etc. The add on's can quickly add up if you need to purchase them afterwards that can kill the deal, so shop accordingly. You can find a complete set for quite cheap, remember winter is coming up so you may have time to shop around. Sometimes you can buy at super silly low prices because they've not been tested but can be worth the risk, I've had good luck with that (sometimes) but I don't necessarily recommend that.
Look at completed sales (prices are in green) to give you a better idea what the avg selling prices are. You may surprised how low it is and sellers are usually willing to negotiate to a lower price if asked.
One thing to remember, the reason why these sell used for so cheap is that there's no warranty and the electronics are dated. These are usually fine to buy used and can be had at excellent prices that make them worth getting but make sure to ask all the pertinent questions, asking if the screen is scratched, faded or crackling from sun damage, if a screen cover is included; if all the buttons work properly and if all the elect. ports are clean and undamaged. Also ask if it's prone to crashing, dead pixels or screen flickers. You would be surprised what some sellers choose not to include in the description. Make sure you buy from someone you feel good about with lots of postings and with good feedback. Their is value in that.
Hope this helps, good luck.