Like all species in almost any invertebrate group, there are some parts of the country that are not as well recorded as others, but generally the SRS situation is much better than for most invertebrate groups, as can be seen on the coverage and diversity maps.
Arachnologists have always recorded all spiders, regardless of whether they are common or rare. Spiders associated with houses and gardens do tend to be under-recorded, for the simple reason that most arachnologists do not invade other people's houses and gardens to survey for spiders.
There is an additional reason we work through Area Organisers and do not currently encourage iRecord - the main objectives of Phase 2 of the recording scheme are focussed on structured autecological data which are not recorded in iRecord. Also, as a separate issue, the current assumption that nearly all and everything can be identified from photographs means that we cannot usually even get adult season data from these records - you cannot determine that a spider is adult without clear evidence that is has an adult male palp or female epigyne structure, details rarely identifiable from a photograph.