Check if there is any other sites around TW. If yes, go check them out and see what they offer. Do your homework and use them when you negotiate with TW. Some might offer to cover stamp duty, free carpets, etc. There are 3 different sites/company when we purchase ours 2 years back, we managed to get stamp duty, basic carpet and basic vinyl. We upgraded the basic carpet & vinyl to premium carpet and tiles.
Avoid purchasing washer/dryer, dishwasher, fridge freezer from them, they are at least 40-50% more expensive compared to buying your own.
Since you have good amount of deposit, i would say avoid HTB.
Also, never use solicitor recommended by TW. Get your own even if they are more expensive.
Good luck =)