You are a Brazilian national travelling with an EU national. You will travel from Germany to an entry port in the UK. Brazilian nationals to not need an entry clearance.
can my German friend join me in the queue of non-EU citizens?
Yes, of course. He can join you in the queue and when your turn comes he or she can accompany you to the primary control point for your landing interview. This is not an overly big deal.
A more interesting question is can you join the EU queue with your friend? This is situation dependent. The Border Force has a policy that families should not be separated if there is any way to avoid it. This allows non-EU spouses and children to use the EU queue with the primary. In your case, you are in a casual relationship (I assume) and not a real family unit. So somebody MIGHT bark at you if you tried joining the EU queue. However, nothing prevents your trying, and if they are not busy, you might succeed. If not, they will send you to the back of the non-EU queue (and your friend can accompany you). This is also not an overly big deal, the controlling factor is whether they are busy or not.
Personal experience: in the days long, long ago when I was non-EU and needed visas and leave-to-enter and what-not, I preferred the EU queue because it was much quicker. If I spotted that the EU primary control point was idle I would just approach it and present my passport. What are they going to do? Scramble the paras? Call out the SBS? About 80% of the time it works. About 20% of the time there is a rigid IO who sent me back to the other queue. It's a great tip for frequent travellers.
having got a visa already in the past can make me go through
immigration in London more easily?
They love to see peripatetic history in a passport. They totally eat it up. However if it looks like you are coming too often there will be problems. In your case, it's fine. Nothing in your narrative raises any flags IMHO.