That's the correct washer but the rubber version is much better.
Readily available any decent plumbers merchant.
If your grant boiler is a combi be very careful not to spill water it drips onto board and there easily killed!
As you have already identified what you have is not a typical set up.
Pump and blending valve normaly being a interagal part of manifold.
Your probably going to need someone eye's on to see it up and running.
The fact your installer fitted normal room thermostats is not a good indication.
Was...
I'm not surprised your underfloor struggles to warm rooms.
To be clear your underfloor should be switched on 24 hrs a day .
You set it to desired temperature then reduce it by maximum of 4°C when room not in use .
The lower temperature is the backset temperature.
Your room thermostats should...
You really should be addressing you concerns about your underfloor with your installer.
You don't operate underfloor on a 'on off basis' you typically need thermostats that let you select a backset temperature,
The backset would typically be a maximum of 4°C below desired room temperature...
Assuming the drops are connected in to part of system that still has enough carrying capacity left
no problem.
In fact standard stuff just cap non needed drops in loft.
First assuming existing waste is from Bath & basin or basin & shower it's already at full capacity and probably pushing the boundaries of needing to upgraded in size.
Add a washing machine and you risk water backing up into lowest outlet in bathroom. Plus you will undoubtedly get gurgling noise...
Pic of your zone valve / underfloor manifold set up would be useful.
As Ben-gee as indicated may be a problem with way returns are connected the return from cylinder must always be last connection.
Use existing pump and 3 - 2 port valves (S plan plus)
You would create endless wiring issues with extra heating pump requiring relays ect.
In a ideal world cylinder return should be last connection before boiler.
First I would strongly recommend a twin coil cylinder with both coils indirect, correct way to do it .
Easy enough to have a bespoke cylinder made .
I use Newark cylinders for special cylinders mostly because I can collect from them rather than pay carriage. They will make a cylinder in any...