Will I, and when will I receive offers? (PPE)

Oxford interview invitations come out around the start of December time provided you have a good TSA and an overall good application. Your 7 at GCSE hurts your chances ever so slightly but that wouldn't be the reason you get rejected if you do. Offers come out around January time if you ace your interviews.
LSE wait for the equal consideration deadline before evaluating anyone, so expect them to update you around march/april time. Offers are contingent on a very good personal statement.
Warwick, KCL are rolling, so review applications as they come in. You can expect notice anywhere from 2 weeks to 2 months, so I wouldn't stress too much on this.

Oxford interview invitations come out around the start of December time provided you have a good TSA and an overall good application. Your 7 at GCSE hurts your chances ever so slightly but that wouldn't be the reason you get rejected if you do. Offers come out around January time if you ace your interviews.
LSE wait for the equal consideration deadline before evaluating anyone, so expect them to update you around march/april time. Offers are contingent on a very good personal statement.
Warwick, KCL are rolling, so review applications as they come in. You can expect notice anywhere from 2 weeks to 2 months, so I wouldn't stress too much on this.

One 7 will not have any meaningful impact on OPs chances

May be true but if you have a look at Section 4 of the admissions report here and look at the fall off between 10 and 11 A* GCSEs, one less A* can take you from above average to the mode result. And if you have a look of the rough percentage of applied vs. interviewed and even applied vs. offered, you can see a meaningful enough difference to point it out. And, when applying to an elite ultra-selective institution, every little advantage you have counts.

What do you calculate those percentages to be?

If I had calculated them I would've said so. However, looking at the bars, just under 1/4th of people with 10 A*s received an offer while just under 1/3rd with 11 A*s got one. ~23% to ~30% jump I would say.