If you put your arm across the end of the seat shell and it touches his head, the seat is outgrown and needs replacing, or if he's 13kg (this is an extremely rare way to outgrow a baby seat, they are designed to be outgrown by height first) or if the top harness setting places the straps more than 2-3cm lower than the tops of his shoulders when he's pulled right down to the buckle.
As others have said, there are seats available in the UK that rear face (or forward face with harness) up to 18kg or 25kg - but it's important to note that these seats are also usually outgrown by height before the child gets anywhere near the weight limit, because of the physical limitations of the harness, top position of headrest preventing taller shoulders from fitting or the height of the shell. These correspond (roughly) with European clothing sizes 104 and 122 - so an 18kg limit seat will usually fit children until about the point where they can no longer wear age 3-4 clothing in most stores, and a 25kg harnessed seat will usually fit children until they can no longer wear age 6-7 clothing. This is a very approximate rule of thumb, but is a useful way to think about it when looking at your one year old, having no idea what size they might be when older.
Therefore it's worth thinking about how quickly your child tends to go through clothing sizes, and whether you'd be happy with changing to a high backed booster seat or buying another expensive Group 1/2 harnessed seat at that age. If you are likely to have a younger sibling to hand the next seat down to and they'll both be in this stage of seat at the same time, or your child is of a size where you think you'd be alright for them to have a HBB when they outgrow age 3-4 clothing, it would make sense to buy an 18kg limit seat now. However if you think you would still prefer them to be in a harnessed and/or rear facing seat at that point, there will (likely) be no younger sibling and/or you just want to maximise their rear facing/harnessed time without buying seats in this pricey stage twice, go straight for a 25kg seat.
Generally the 18kg ERF seats can be bought in typical retailers like Mothercare and John Lewis, but the 25kg ERF seats are available directly from specialists (recommended, as they will give you a face to face tutorial in fitting and evaluate the seat for your car/child) or online only.
For me although I think DS2 will probably outgrow the 18kg seat by about 3.5 years old, I really want the proper recline you get with an 18kg seat and as we do (very) long drives a couple of times a year I think that's more pressing. We might have another child in need of the seat by the time he outgrows it, in which case I'll get a 25kg seat, but if we don't, I'll probably just get a very sturdy HBB with crotch strap.
For the poster with the Silver Cross Simplicity, did you know how to access the hidden harness adjustment part?