I'd say that voice would be enough for A level as you can record with just one instrument. However, when I decided to do music A level it was advised that you were already at a grade 6 standard. Luckily I was grade 8 but a distinction at grade 5 sounds promising. I'd also say that it's a really good thing that you know the basics of the piano as this helps when working in the bass clef and chordal writing in the compositions. The only problem I have is memorising all the information about the set pieces. At AS I had to learn all about Haydn's symphony 104 and about Baroque choral music. There's a lot of information to remember for these and it only increases when you get to A level.
For my spec at least (idk about new spec) you can choose to do two technical tasks (chorale or counterpoint) or two compositions, or one of each. Technical tasks are easier in the sense that they are more prescriptive but it might be more difficult to get absolute top marks versus composition. However, the composition is more vaguely graded and often where people lose out significant marks (eg average A but composition a C/D) because of the disparity in teaching quality and the fact some centres "help out" their students a lot .
The written exam involves learning about the features of six instrumental set pieces and 5 applied music pieces (again at least for my spec). The applied music questions ask you about the style of the music and you give features with references to bar numbers/locations (two of those both out of 13). The instrumental question is a 3-three comparison so you compare say structure and harmony in [piece 1], [piece 2] and [piece 3], using comparative words (similarly, in contrast etc) and musical features with bar numbers/locations. The amount of info for the applied music isn't too bad and the essays for those are straightforward - the instrumental music has a lot of content to remember but the structure is also straightforward.