Welcome.
The 463 is a model that has been superseded by the 473 which retails at £400 & I know it is well above the budget you quote but is is vastly superior to what is shown below. The only model which imo is superior to the 473 is a Korg EK50 which retails at about £300. As your son will be used to playing the 463 it would make sense to up your budget and to stick with a Yamaha and the 473 will provide years of service,
I would suggest looking for a second hand unit but the likes of eBay sellers usually have free collection as it can easily cost over £50 in postage and in any case what is on the site do seem to command a high premium. You can always ring Music Stores and ask if they have any Trade In keyboards.
There is the Yamaha PSR E273 which retails at £159 from Bonners and £163 at Rimmers.
The image is a screenshot of what is available within your price range from Rimmers Music & as you can see what there is is very limited.
Notice the Casio’s, I recently bought my Grandson one for his birthday and I tried it before, for his dabbling and non committal attitude to learning it was more than enough cash to part with. That aside it is in reality little more than an expensive toy with so so sounds, best avoided for someone serious about learning.
