Essentials TV does not include any channels. You can get Sky and other channels by subscribing to boosts but you won't be able to time shift the programmes as the Youview box provided with Essentials TV does not record. The on-demand catch up service is pretty poor - 99% of what's on is not available via catch up. Many subscription channels (e.g. Syfy) don't even have a catch up service on TT. The Plus TV package does give you a Youview box that can record, but it's limited to a single subscription channel at a time. A while ago you used to be able to watch one while you record another but they have stopped that too.
The other thing to think about is how good you internet is in terms of speed and stability. Mind is fast enough and is rock solid, except between 8-9pm every few days when the connection willl drop for a minute. As it is prime time for watching TV it is very annoying when it happens.
If you just want to take the offer to save some money, then do it - it doesn't really matter if the TV is any good. If you want something that matches Sky, then you are likely to be disappointed.