As I said before if you are mixing the RO water with HMA'ed RO waste water there is no point in having the RO unit.
1. The RO pre-filter will remove sediment, chlorine, chloramine & organics (same as HMA filter will). Chlorine and organics will degrade the RO membrane.
2. The output of the RO will effectively be pure water.
3. The RO waste will contain the carbonates, nitrates, chlorides & metals (mainly magnesium and calcium).
4. Feeding this via a HMA filter will remove the heavy metals, leaving the carbonates, nitrates, chlorides, magnesium and calcium. The HMA would also remove chlorine and organics, but these have been removed by the RO pre-filter.
5. You mix RO and HMA water. You now have water with same levels of carbonates, nitrates, chlorides, magnesium and calcium as you source water, just no chlorine, organics (filtered by RO filter) and no heavy metals (filtered by the HMA filter).
You could therefore achieve exactly the same using just the HMA filter.
In fact even cheaper just use UK tap water and any good brand dechlorinator. As well as taking out chlorine/chloramine most will remove heavy metals as well, not that heavy metals in UK tap water is ever an issue.