My understanding is that it’s likely that the Autumn booster jab will be one dose of a different type of vaccine to your original one, I haven’t heard anything to suggest that if you have only had one dose of a particular vaccine you will be offered a second dose of the different type as part of your initial vaccination regime, so the recommendation is still that you need both doses of your original vaccine.
So for example, if you have had one dose of Astra Zeneca, that only gives you partial protection. Having one dose of Pfizer after that would still not give you sufficient protection, but if you have had both doses of one type, one dose of the other will potentially boost your immunity further. I may be wrong, but that’s what I’ve understood the scientists are looking at.