Boiler and unvented tank positions

It would be more ideal for someone to come and have a look. but a g3 qualified unvented engineer can possibly talk through the options. Tanks would then not be needed in the loft space leaving you with everything in the garage

You can put the unvented in the loft, with the boiler on the ground floor. This is the set up I have... however, make sure the flow and return are as best insulated as you can do it, otherwise you are looking at a lot of heat wastage. You will also need to confirm with your cylinder supplier of the siting position as there is an additional safety feature requirement required. Finally, you will need to think about the discharge pipe from the safety valves. This will need to be 22mm copper and ideally terminating outside into a suitable drain. It is allowed to feed into waste pipework but not preferable... worst case a third of your cylinder may need to be dumped at perhaps 90 degrees - not ideal into plastic pipe!