If you have resigned and not been dismissed for gross misconduct, you have a notice period - in your case, its four weeks. Both parties have to honour this.
Your employer can ask you to leave the work place in advance of those four weeks but you have to be paid for them - its called gardening leave.
If you have another job to go immediately into, you might want to be released immediately because you cannot work for another company while you are on gardening leave for your (soon to be) ex employer, but if you don't, you have to be paid for the four weeks.
So, first thing to do is to all your employer - HR department if there is one - to clarify the situation; I would say you appreciate their position about them wanting you to leave now, but that you want to confirm what your final payment will be and want this in writing. This will be the four weeks of your notice period plus any holiday entitlement you have accrued that has not been taken OR minus any days of holiday that you have taken but have not accrued.
If they say they're not going to pay you and they have NOT dismissed you for gross misconduct, what they have done is unfair dimissal - you should remind them that if you take that to an employment tribunal, you win automatically, regardless of the disciplinary circumstances (personal vendetta or otherwise).
If you do want to go immediately, you can try to negotiate this but do rememember that they have the right to make you stay on gardening leave for the four weeks if they want to - it might not seem fair and doesn't make much business sense if all they want you to do is stay at home but if they're being difficult, they are within their rights to do this.