i figure there are exceptions with some spare tyres if they are only used temporarily
, like to get you to a mechanic/repairer. for example, cars fitted with 'space saver' spares often have speed ratings quite low (ie: max speed of 80km/h), and the wheels/tyres are marked accordingly. this is obviously legitimate to use.
for you 'proper' daily-driven tyres, both the speed rating AND load index must be the same, or higher than factory specification. or at least, here in australia, both these standards must be met, so i figure it's the same in the UK. some people overlook the load index, mostly cos they don't understand what it is, or that it exists. you don't really have to understand it, but simply know that your tyres must not be below the specified number.
for example, a 156 with a V-rated tyre, probably also has a load index requirement of 88 or maybe higher.
these specs are generally very easy to comply with; it would need to be a rather cheap and low quality tyre not to. it becomes a little more difficult if you go with a lower profile tyre (on a bigger diameter wheel); low-profile tyres tend to have a lower load index, simply because of physical limitations in their construction.