When I was a Thameslink customer I found that (except just before a big fares increase) it was more economical to buy around nine monthlies over the year, extending each one by two or three days; so that each ran from the Monday of week 1 to the Friday of week 5. This works because the two or three add-on days are each charged at 1/30 of the monthly cost, ie not based on ~22 weekdays in each month.
Tweak the dates to avoid having a ticket that covers Bank Holidays (unless you intend to travel that day). And of course, time it so that you don't have a ticket that covers your annual leave (4+ weeks?), or distant work postings/courses.
And if during the year you have a lifestyle change (new workplace, long sickness), your financial exposure is much less; no faffing about with a (poor value) refund of your annual (they charge you at the monthly rate for the period used, not pro rata of the annual sum; hence a zero refund after Month 10).
And of course with this "Nine Long Months" plan, the TOC doesn't get an interest-free £4600 loan from you
.