The FTSE Small Cap index consists of companies that are in the FTSE All-share index, but are too small to qualify for the FTSE 350.
FTSE also publishes an index of companies that are too small to qualify for the FTSE All-share index. This is the FTSE Fledgeling index.
The remaining FTSE UK small cap index is the All Small index which combines the small cap and fledgeling indices.
Like other small cap indices, these are much less widely followed than the large and mid cap indices. The reason is that the bulk of market cap is in the large cap index (in this case the FTSE 100), and most of the remainder is mid cap (FTSE 250).
This is also why the broad indices such as the FTSE All-Share are little used. The addition of small caps makes little difference: at most they will make the index very slightly less accurate an indicator by diluting movements through the addition of illiquid small caps.