1, No it they will not reduce pressure at all. they will reduce flow slighlty but on a combi it will not make the slightest bit of differece to your shower/taps. They are fine to use.
2, As mentioned do not use gate valves. They will be fine for a few years but will in time either start leaking, not shut off or both. They are not designed for mains pressure.
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local