What do you want the unit to do? Obviously you want it to store your TV, but what else does it need to store?
What does your current solution do that you need to replicate and what is it missing that you need to have?
Consider sky box, amplifier & other AV boxes, playstation, sonos, books, drinks, glasses, vases, photos of the family or any other display items.
Measure the above items (in cms) and make a written list.(it may seem like overkill, but if you do this you will end up with a much better solution).
2. Sketch It
Do a rough sketch of your room. It doesn't have to be architectural but make sure you include plug points, radiators, window sill heights and any other fixed items on the drawings. If there are lose bits of other furniture that you are 100% certain you'll be keeping in the room, draw them in too.
Measure everything in cm rather than inches - its easier when selecting items online or in shops.
This will help ascertain the best storage options and reconfirm the best position in your room for your unit.
If you're feeling up to it, try using a free drawing program like Sketchup to get more accuracy.
3. Moving your TV / data points is not difficult. Don't automatically assume that you have to position the TV where it has always been because that is where the points are located. If you figure out a better layout / position for the room / TV unit, getting an electrician to move a few points around is really not a big deal.
Speak to the in-store designer to get more advice on this. Very often the company supplying your unit will do this for you at a small charge.
4. There is no such thing as "we don't have a budget". Rather than setting a fixed figure like £2000, better to set a range. Be clear with yourself what your range is. What is it that you expect from your range? If you're looking for a large 4 metre unit, with wall to wall cabinets, a bar unit and disco lighting, will your range realistically cover it? Make sure you set a realistic range before and during
your online research
5. Take lots of photos of the room where the unit will be placed. Take a video too. Don't just take photos of the wall or corner where the unit will go - be sure to get a complete 360 of the room.
6. Save time by looking online before heading out - look at online customer reviews of the shops you are thinking of visiting - there are many shops that have fantastic reviews - aim to see those first. Avoid those shops that suppress or don't encourage customer reviews.
Obviously also avoid those that have poor reviews!
7. Create a folder and save some images in it. Try https://www.pinterest.co.uk/ - type in "TV wall units" and save some of the images that you like the look of. That will help you find your solution quicker when you get to store.
8. Plan your hunt - make a list of stores that you want to visit - speak to the store beforehand to understand their price points to ensure they are aligned with your budget range. You need to see as many units as possible when making this decision - find out how many units the shop has on display before adding it to your visit list. Anything over 10 displays is usually a good start
9. Make sure the store has a design team available to help you design your unit. Take your photos, lists, room sketch and (Pinterest) saved images with you to help the in-store designer.
10. If you can, ditch the kids. Send them off on a play date or drop them off to relatives, friends, babysitter, zoo - anything to give you a few free hours where you can focus.