Anyone considering becoming a commercial pilot needs to realise that it is especially unlikely in today’s/future world that you will ever make the £100k back. Just consider inflation alone and the possible lost compounding effect from missed investments. You also need to realise that even if you are earning £100k, you pay a lot of tax, then you’ll have mortgage/rent, pension plan, insurances, food, children etc etc and possible costs of commuting(paying for hotels or a second place); it’ll take many years just to make the base amount back. If you are a single income household, £100k doesn't go too far these days if you aspire to own a decent home and save for retirement, especially if you want to live in somewhere like the south of England. This is real life I'm talking about not the delusional starry eyed teenage dream of a hedonistic career in aviation.
Its also worth mentioning that incomes across the entire workforce have been eroded through inflation over the last 20 years, there is no good pension schemes left out there for new joiners and the cost of living only ever increases. What does the next 20 years hold in store for us?
If you can manage a few years as a skipper in the Middle East, pick up an ex pat contract in China or make it to the top of a seniority list in a flag carrier; then you might get a return from the £100k cost of your licence.
Think very carefully about investing any money into a career in aviation never mind £100k. Only do this job if its your passion and you have the aptitude, otherwise avoid it like the plague!