JSFiddle - React, Tailwind, and code Playground

HTML

<input id="income" type="text" value="£"/>

JavaScript

$("#income").keydown(function(e) {
    var oldvalue=$(this).val();
    var field=this;
    setTimeout(function () {
        if(field.value.indexOf('£') !== 0) {
            $(field).val(oldvalue);
        } 
    }, 1);
});

  // Add comma to input field
    $('#income').keyup(function(event) {

    // skip for arrow keys
    if(event.which >= 37 && event.which <= 40) return;

    // format number
    $(this).val(function(index, value) {
      return value
      .replace(/\D/g, "")
      .replace(/\B(?=(\d{3})+(?!\d))/g, ",")
      ;
    });
  });