It is still early days in the application cycle until Jan deadline. Bath may receive a late influx of applications closer to the deadline. I also think that the sample size of peers does not reflect Bath's true standing. Bath competes more with Loughborough and Exeter. UCL, Warwick and Bristol have better national and particularly international reputation. This leads to a cross-over win for its rivals. International applicants may look at THES, Shanghai and QS rankings where Bath is lower ranked than UCL, Warwick, Bristol and Birmingham. Psychologically, most international parents may prefer better known unis if they have to pay prohibitive overseas tuition fee. This explains why these unis outperform Bath relatively in application numbers. Another factor is Bath's location and relatively higher cost of living. With exception of UCL, the other unis are in cities with lower costs. Baths regional coverage is mostly the South with mostly middle class demographics. I do not think that a pupil from moderate background from the North would apply to Bath. In some subjects, Bath receives moderate student satisfaction in some surveys. Bath had places in some subjects in 2017 clearing.
There are some short term tactics to boost numbers. As mentioned above, unconditional offer if firmed 1st choice. Contextual offer for disadvantaged domestic applicants, outreach with school visits and organised open days, aggressive international recruitment abroad combined with softer entry requirements, working with admissions consultants abroad. Some tactics are controversial and Bath may not use them.