I wouldn't talk about "guarantees" for interviews, as for most people this is not a thing. Some people might have guaranteed interviews through certain schemes or widening participation, but for the majority of people you raise your chances through your UCAT.
Different universities have different requirements and weighting for each aspect when deciding who is invited to interview. Some will entirely be UCAT, some will be a mix of A level/GCSEs and UCAT weighted in various ways. You have to research each individual place. It's best to not obsess over any particular university until you have sat your UCAT and received a score, as the score determines a lot of what will be realistic/a good option to apply to. A high UCAT score can compensate for lower grades and vice versa, depending on the institution.
In general extracurriculars aren't important for actually being invited to interview as this would be disadvantageous to candidates who struggle to arrange them or don't have access. What they are good for is giving you experiences to reflect upon in the interview. However, the quality and depth of your reflection matters a lot more than having a larger number that you give a shallow reflection on. Better to focus on one to two proper experiences and go into depth with how they can be used for interview scenarios.
In conclusion, you want to sit your UCAT this summer aiming for the highest score you can get as this opens options for you. Try to get some volunteering experience, but don't worry too much about how much you do, focus on trying to reflect on what you learnt and how applicable it is to Medicine. After you've got your UCAT score you can start investigating what would be realistic options you could apply to to get an interview.