What is my position on being stopped instore?

I was stopped recently by a Lidl security guard who is known to be over zealous and I did not like his unprofessional attitude either. What can I do if I want to refuse a search of my bags and then walk out of the store. This is in the UK.

I did not even pick up anything as they were out of stock.

Short A: Whilst you have a right to refuse a search or to answer questions you do run the risk of being arrested depending upon all the circumstances.
Further explanation: Your Q, and A's, seems to revolve around what is commonly called a citizens power of arrest and search.
With the greatest of respect to all those A's who tell you they can't do anything until you leave the store that is not strictly legally correct. The reality is that a customer is normally allowed to go past the available pay points and even through the doors as it is easier to prove the intention to steal BUT. Theft includes various ingredients, elements of dishonesty, appropriation of someone elses property and an intention to permanently deprive the owner of it. These ingredients may be proved at various times, some actions being more obvious than others. Swapping price labels, removing packaging, concealing garments and items inside other articles etc. etc. might all be proof of theft and be clearly shown whilst a person is still inside a store.
Next issue is the right to detain / arrest. Simplified, provided an arrestable offence ie. theft, has been committed, any person - here read store security, may detain any person they reasonably suspect may have committed that offence. This means that provided articles have been stolen they may actually lawfully detain the wrong person if the person they detained was reasonably suspected as being the right person ie the thief.
As in all things what one person, especially an innocent and embarrased shoper sees as over zealous the employer may see as reasonable protection of his goods.
So to try and tidy these ends up, if you feel annoyed at the way the situation was handled then you have a right to try and speak with the management, escalating it up as far as you feel justified if unhappy with any initial explanation.

Technically, for citizen powers I should perhaps have used the term 'indictable offence' & not 'arrestable offence' - see also Serious Organised Crime & police Act ( SOCAP ) in force Jan 1st 2006 for extended Police Pwers. Report It

You cannot be stopped inside the store as you can always say you were going to pay for an item, they must let you get out of the door. Then they can stop you and demand to search your bag, if you refuse they have the right to call the police.
Tell them you want to see the manager and complain, saying you have not got anything from the store and hadn't left the store when the security guard stopped you, making you extremely uncomfortable.

If you do not want your bags searched refuse to allow it and keep walking.

If you have stolen something then the security guard can detain you but in order to prove theft you would need to have passed the last point of payment, usually in practice that means leave the store.

If you don't like his unprofessional attitude why not take your business alsewhere?

Refuse the search

You have not comitted an offence untill you leave without paying.

If he grabs the bag or yourself, call the police. (on your cell phone, report an assult)

There will be CCTV in store to back you up.

If you don't have a 拢1 coin spare, you often need to use your own bag to put your shopping while in the shop (before paying). There are no baskets.

my understanding is that for them to prove theft you have to be outside the store, because theft is defined as intent to permanently deprive. If you're still in the store they can't prove that intent - you might have put the thing in your bag because, say, your basket was full.
I would have thought you could refuse a search but I'm not sure

They cant do anything inside the store, you have to be outside, as in youve taken something out of the store without paying for it. If the security guard is stopping and trying to search you inside, he has no right to. I would see the manager of the store and tell him/her, and make a complaint.

I'm pretty sure they can't challenge you until you leave the store as there is no law against carrying stuff to the tills in your pocket/bag. I think you can refuse a search of your bags until you leave and then you can be stopped and held until the police arrive.

you have the right to refuse to let him search your bags inside and outside the store, he then has the option of calling the police, if you have not stolen anything you can sue the shop, you will get 5 to 12k.

Speak to management, and explain this makes you feel uncomfortable, and they shouls sort it out.

They can't search your bag without your permission. I would write to the Chief Executive of Lidls and say how disgusted you are.