you don't need train tickets for April until a few days ahead for the cheapest. Tap and go if you don't expect to benefit from the 2 for 1 discounts, or you can even just walk up and get a ticket. Walk up and tap and go will be more expensive than an Advance a couple of days ahead. You don't need to get them in January. The train doesn't sell out, there are no reservations available.
I didn't look where your hotel is, but Chelsea is not difficult to reach Euston Station which is where you will have to get to.
Follow up
I've had a look for Hotel Pestana Chelsea as you name it, and I come up with Pestana Chelsea Bridge Hotel & Spa. Is that it? If so, it is NOT in Chelsea but across the River in Battersea, across the Victoria station train yards and tracks from the Battersea Power Station.
The easiest way to Euston from there is to get to the new Battersea Power Station Northern Line tube station on the new branch of the Northern Line. Get off at Euston and go upstairs and you are there.