All 'primary' legislation has to be passed by both Houses (except, as stated, when the Parliament Act is used).
However, much of the legislation which is thus brought into effect includes 'enabling' powers, permitting the relevant Secretary of State (or other relevant authority) to sort out the finer points of the law by introducing Regulations (which have the effect of law). For example, the Health Act bans smoking in enclosed public spaces but doesn't provide an exact definition of 'an enclosed public space'. It simply includes provision for the Secretary of State to decide, through Regulations, exactly what constitutes such a space.
Similarly, new legislation no longer refers to, say, 'a maximum fine of �1000'. (If it did, Parliament would periodically have to spend time updating the legislation, through both Houses, to keep pace with inflation). Legislation now refers to, say, 'a maximum fine at level 3 on the Standard Scale'. The Criminal Justice Act 1991 provides the Secretary of State with enabling powers to determine the maximum amounts which shall apply to each point on the Standard Scale.
There are sometimes 'grey areas'. The courts have occasionally ruled Regulations, brought in under enabling legislation, as invalid, requiring the Government to introduce primary legislation.
Jacqui Smith clearly believes that enabling legislation gives her the powers to amend UK visa rules, without the need to introduce new primary legislation. It's likely that she's right when it comes to simply reducing the maximum stay, for non-EU passport holders on a tourist visa) from 6 months to 3. Human rights lawyers may well try to challenge any decision to introduce a bond system through Regulations, rather than through primary legislation, on the grounds that the Home Secretary will be exceeding the enabling powers granted to her.
Chris