I'm replicating people a bit but hey, ... on wards I've been on, no-one uses the titles F1 and F2. Instead the F1 is called the house officer and the F2 is a senior house officer. But then ST1-3 (?), i.e. people that aren't yet registrars are also called senior house officers.
On some wards, the house officer does almost exactly the same as the senior house officer, bar perhaps the house officer must update the patient list. This means that all (or most) of the jobs get divided between the HO and SHO. This seems to be more in 'medicine' wards than 'surgical' wards.
On other wards, the SHO never seems to be around and the HO has to do a million jobs without much senior support.. this is more likely on surgical wards. I imagine many teams are somewhere in between.
SHOs have more clinics.. on my surgery firm, whenever the SHO, reg and consultant have outpatient clinics, the HO has pre-assesment clinic- for patients pre-surgery.
As a nurse, the HO and sometimes the SHO are the most assessible to you. The HO is the person you annoyingly get to rewrite your drug charts or write up more painkillers or redo cannulas.. etc. The HO has to do a disproportionate amount of work generally, especially when the SHO is unhelpful. They both may attend theatre but for the HO its more optional than necessary.