I'd complain to them. Explain that it's there mistake and you should not be held liable for something which was out of your control. If they brought out one TV from the store area, it's not like you picked up the wrong one.
Also Say that your busy and if they want they can swop the item over bt as you work they will have to do the leg work, meeting at your place a a given time, late in the evening is normally good. They can swop the item over. (leaving you the new items box, and take the old one away un packaged). Do this after you get home from work, say 9pm on a friday evening. They can't make, there fault.
Insist that the new one is connected and configured fully, and explain that the matching items now don't match so you'll be wanting compensation for the un-matched items.
And then to finish is off, say that you don't like the tone of the letter, and also are concerned that they got your name and address. You would like to know how they got this information, and then what right they have for using this information to chase you for additional payments, in a intimidating way. Ask for the name of the company Information officer and head office complaints.
They would probably back down "as a gesture of goodwill" as the £200 is not worth the effort for the amount of time they will have to spend.
Or you could just write to them saying you have taken note of the letter dated ?????? and that you are concerned where they got the name address and sales information for. Say that you were not aware of giving permissoin for these details you probably gave over when buying the TV to be used in this manner and that you require the company to cease processing and storing of any records that hold your address. Say you withdraw permisson for them to retain this information and then say if they continue to hassle you, your going to report them to the data commisioners office as a breach of the data protection act.
Or, just ignore the letter and see what happens.