Add template block around table header in basket_totals template for easier overriding
<div id="basket_totals">
<table class="table table-sm">
<tbody>
+ {% block basket_header %}
<tr>
<th colspan="2"><h3>{% trans "Basket" %}</h3></th>
</tr>
+ {% endblock %}
+
{% with offer_discounts=basket.offer_discounts voucher_discounts=basket.grouped_voucher_discounts %}
{% block discount_totals %}
{% if offer_discounts or voucher_discounts %}
