I want an Apple appointment to clean my MacBook storage

Maybe we can better help you—even help you avoid a trip— if your expand on the issues for us:

1) How much storage did your Macbook Pro have when new? Commonly they had 128GB or 256GB as the base.

2) How much available storage is currently available to you? You can quickly find out by highlighting (single-clicking) your hard drive icon and then pressing the key combo COMMAND i to get this:

3) Did you encounter a problem due to low available space, like being unable to apply an upgrade or install an app?

This Apple article has suggestions the novice user can do to optimize storage:

Free up storage space on your Mac - Apple Support

While an Apple Store might not offer the service you are looking for, a third arty Apple Authorized Service Provider will likely offer to help you with this (for a fee, likely the fee will be based on the time required to help you).

You can find these third party Apple Authorized centers by searching in Google: I entered London Apple Authorized Service Providers and many answers came up. You can pick the one most convenient to your location.

I don’t think Apple has staff available for such assistance to customers, beyond a basic sales talk. You may try to find an independent consultant who can do so, but their usual clients are corporations, and their fees are likely out of reach for individuals.

My personal recommendation is to buy a MacBook Pro with plenty disk capacity for any current workloads. But don’t use internal disk for archiving or similar non-current projects. Consider storing what doesn’t have to be available in the moment on an external disk or network disk. Then double that for backups. A consultant can tell you something similar, but you have to do that offloading and shifting, routinely.

I bought an 8 TB external disk in 2013, which still serves me well (52% filled now). I would buy bigger capacity, if I were shopping today, as I seem to use bigger files each year. Your use will be different from mine, so you may want/​need a different capacity, but get “more than enough” to future-proof yourself.

To add to steve’s post,

This link helps you find Apple Stores and Apple Authorized Service Providers (AASPs):

Find Locations

Is it correct to infer, you do not what to perform this action yourself ?

Below may sound technical and daunting for some but it is entirely doable at not cost

Manage Storage

It is generally a good computer practice to alway keep at least 15% to 20% of the Total Drive Capacity’s as Empty Space.

Allowing the computer to drop below these guidelines may eventually cause unintended consequences.

There is Purgeable Space and there is Empty Space.

Purgeable Space which is Controlled by the Operating System. When the Operating Systems decides the computer needs additional Empty Space, it will move a portion of the Purgeable to Empty space

AFAIK - there is no User Actions to hasten this transition from Purgeable to Empty Space

It can day or longer before this will occur  

The links below will assist in identifying what is taking up space on the Internal Drive and provide possible ways to remove data that is under the direct control of the User ( Home Folder ) . 

Rebuild the Spotlight index on your Mac

What is “Other” storage on a Mac, and how can I clean it out?

Free up storage space on your Mac

OmniDiskSweeper Safe to use

GrandPerspective 

How to delete Time Machine snapshots on your Mac.  

Often caused if the Time Machine Drive has not been attached  to the computer and TM Backup is set to run on a Schedule. 

TM Backup will make Snap Shot on the Internal Drive awaiting the TM Backup Drive to be attached. 

Only then will the Snaps Shots be transferred to the External Drive and deleted the Internal Drive.

See used and available storage space on your Mac

Locate backups of your iPhone, iPad, and iPod touch

Notation - If the user is using a cloning software like Carbon Copy Cloner - suggest tweaking the Safety Net Feature in this software. It may be making additional Snap Shots that are not being Cloned to the Eternal Drive. If this should be the case, these Snap Shot could be using additional space on the drive 

The final word from Apple on Managing the " Other/ System Data “ Category

Other / System Data: Contains files that don’t fall into the categories listed here. This category primarily includes files and data used by the system, such as log files, caches, VM files, and other runtime system resources. Also included are temporary files, fonts, app support files, and plug-ins. You can't manage the contents of this category. The contents are managed by macOS, and the category varies in size depending on the current state of your Mac.