I don't live in the UK so not sure what is available there. But do know this, there isn't a lock made that a battery powered angle grinder can't make short work of...well, there is a new D lock on the market that an angle grinder can't cut, but it's freaking HUGE! Not something you would want to carry around. Google: Altorlocks SAF lock.
However, most bike thieves do not carry angle grinders, a pro might, but they're looking for specific bikes with expensive components, if you have something like that then best not to park it someplace where it can be stolen. If you have an average type of bike and want to make sure when you get off work, or out of class, or out of a store, that the bike is there when you come out, and don't want a great deal of heft to carry around while at the same time not costing as much as the bike you ride, then try this.
Get a Master Lock model 1175RS combination lock, it has a hardened Boron Alloy 1 1/6th inch long shackle that gave a workout to angle grinders, but they did manage to cut it, it just took a while longer than other shackles they tested, and a large bolt cutter could not cut it.
Next get a Abus Security Chain 6KS. The 6KS is the smallest diameter (1/4 inch thick) chain link they make which is good for helping with keeping the weight down, also the links are square not round which is more difficult to get a bite on, but it's still quite stout of a chain, enough so that a large bolt cutter could not cut it. Abus does have thicker chains, the 6KS is their thinnest chain, but with thick links comes more weight which will make it bad to carry around. You can also have the chain custom cut for the size you think you need, they come either 2', 6' or 10', but you can get a custom length of any size, I got a 5' chain, and almost wish I got a 4' instead, but at least I can use the length to wrap it around almost anything.
The weight total of those two items is around 5 pounds for a 5 foot chain including the lock, but the cost is only about $50 for the chain, and the lock is about $30, so for $80 you can have an extremely tough locking system, and with a combo that you set the combination you don't have to worry about losing a key.
Just a suggestion obviously. There are a lot of different bike locks on the market, some cost a lot of money, but a lot of them promise the moon but do not fullfill like the Hiplok Anker that a angle grinder cut through as fast as it did with the system I mentioned except it cost about $180 more! And that Hiplock was the strongest of all the designated bike locks they tested so it got a Gold Secure rating. The Hiplock can be worn around your waist like a belt, I suppose you could do that with any chain lock, I simply put mine in my backpack. LiteLok is another that got a Gold rating, but a guy on YouTube cut through that easily, see:
View: https://www.youtube.com/watch?v=D-On0DGcDlc
All the locks I looked at with a high rating weighed at least 5 pounds, most were over that, and cost at least $120, but none offered the flexibility that a chain allows you, a chain can be wrapped around and though anything, D locks...no, you're limited to how to lock up the bike and to what you can lock it up too. And all the bike specific locks didn't take much effort to defeat them, the strongest one was the Hiplok Anker which put up as much fight as the pad lock and chain I got.