Invoking itself will have zero direct impact, by then everybody and their dog will have prepared for that to happen. It's why we see the £ drop today rather than later. One main problem at this point is unknowability (is that even a word?)
of the future situation as nobody has even a reliable idea what agreement will be reached. Companies can plan for, say, a scenario with 10% or 20% EU import tariffs but you can't plan for the unknown. That renders most investment profitability calculations moot (including but not limited to the internationals), which may well mean postponing the investment or transferring it to another area (into EU territory in this case).
Shifting investments abroad means less investments in the UK, which translates to a weakening or actually weak economy. A weak(-ening) economy usually begets a weakening currency, for various reasons.
UK government members are trying to prevent these shifts, among other things by making big promises to Japanese far manufacturers. Some of these promises may well turn out to be untenable or be extremely expensive.
Not sure, but go the Donald (Rumsfeld) way, the Known knowns and the unknown unknowns, fat better turn of phrase !