A quick look at two types of 5 amp fuse, BS1361 and BS1362.
BS1361 fuses are designed for use in consumer units, the 5 amp version most often being used for lighting circuits.
BS1362 fuses are designed for use in BS1363 plugs and fused connection units. These are slightly longer than the BS1361 type, and attempting to shove one of these into a consumer unit fuseholder will usually result in the fuseholder being damaged or destroyed.
Both BS1361 and BS1362 fuses are available in other ratings, however this problem of near identical size only occurs with 5A types. All BS1362 fuses are the same physical size, but BS1361 fuses are physically larger as the rating increases.