There's no hard and fast rules set by HMRC for the value at which a purchase becomes a capital item versus when it can be expensed as a day-to-day running cost of your business. However it generally tends to depend on the size of your business. For example a £32 laptop stand would probably be classed as a day-to-day running cost for both a small and big business. Whereas a £320 phone system for a small business would be a capital item, but probably a day-to-day running cost for a larger organisation.
It's a judgement call that's up to you as to what value you set for items to capitalise or not but generally speaking things such as the following would be treated as assets:
MacBooks or Personal Computers
Other high value electronic equipment
What's not included
Laptop stand, HDMI cord, computer mouse and keyboard are all considered to be quite low value items. Depending on your company policy it might be a good idea to set a limit as to what you consider a capital purchase or not. Anything below that limit you can expense straight away.