This all depends on who you are designing and supplying to. If it's cheap budget bathrooms then yes you can make a single visit, chuck some brochures their way and let them guide YOU. There isn't going to be a huge amount of stuff they can choose from so it's nice and easy, you can then send them off round the local tile shops to choose their tiles within budget. You get it all in, make a small mark up and off you go. Remember it has to be a small mark up because they are on a budget.
But if it's for somebody that is willing to spend money and I’m not talking huge money, just somebody that values 1 firm taking responsibility, then it's very different. 3, 4 even 5 visits aren't uncommon. 1st visit for the measure up and a chat about their likes dislikes, 2nd visit you showing a few different designs, 3rd visit firming up on a design and showing samples of tiles you've chosen, the bathroom suite you've spec'd, 4th visit to confirm everything, arrange a start date and take a deposit. Make no mistake it's a lot of work but then you should be charging a premium for all that running around, all that time spent researching and keeping up to date on what hot and what's not. That time has to be paid for but then you have to offer so much more than just turning up and doing a day’s work. In order to make that pay then you have to build your design costs in to your price, that means adding an extra £15 sqm on to the price you pay for tiling, an extra £150 on the plaster. the sparks, the decorator, 25% on materials.... And always keep in mind that at any time they can pull the plug on you after doing 3 days of design work only to go with another firm, so it isn't just building in your time for that bathroom but also the ones you lost.
You also need to think about your suppliers. Clients are willing to pay a premium for a bespoke service but you still need to be competitive. So you need to look at getting you suites either direct from manufacturers or distributors to allow you to build that profit margin in, same to a certain extent with tiles. Don't forget it is you that is doing the designing so it is you that guides them and uses products from the suppliers you deal with. Although that said you better have some bloody good suppliers in all price ranges to suit the project in hand.
So it's no easy game taking the whole thing in house. If it's budget bathrooms, the 3,4,5k's you can expect the client to do some of the running around under your guidance, but if it's 8-15K+ then it's a different ball game, cash rich, time poor type of clients.
If you're not prepared to do the running around and spend a considerable amount of time on the design and pricing then you have 2 options. Stick to budget bathrooms or spend £60k on setting up a bathroom showroom.