I don't think your approach is unusual at all tbh. It just feels less common probably because of the 'prestige' and competition for places at these big firms. If I end up having to make a choice between one of the bigger firms I've applied to and one of the boutiques that is very specialist in what I am interested in (e.g. Peters & Peters), it's going to be an absolute no brainer for me. I value having a quality life as much as a quality workload, and I understand the exit opportunities are pretty much the same anyway if you're good enough.
Also, FWIW there are a few firms that are a bit less transactional than K&E if you look closely. Brown Rudnick, which I'm interviewing at tomorrow, is technically full service, but has a huuuuge practice in contentious insolvency, regulatory and corporate crime work. I think
Arnold & Porter and Covington are also very focused on IP and in particular life sciences in London, which is more advisory and contentious (at times) than transactional. Although it does have a formidable reputation for corporate/finance work,
Skadden also arguably has one of the strongest arbitration practices in the UK - it has 3 QC's!