gistfile1.txt

// Add text before sale price only
function bd_sale_price_html( $price, $product ) {

  if ( $product->is_on_sale() ) :

    $return_string = str_replace( '<ins>, '<ins><br>Sale Price: ', $price);
    return $return_string;
  
  else :
    return $price;
  endif;
  
}
add_filter( 'woocommerce_get_price_html', 'bd_sale_price_html', 100, 2 );

WooCommerce: RRP & Sale text in front of price

// Add text before regular price and sale price
function bd_rrp_sale_price_html( $price, $product ) {

  if ( $product->is_on_sale() ) :
    $has_sale_text = array(
      '<del>' => '<del>RRP: ',
      '<ins>' => '<br>Sale Price: <ins>'
    );
    $return_string = str_replace(array_keys( $has_sale_text ), array_values( $has_sale_text ), $price);
    
  else :
    $retun_string = 'RRP: ' . $price;
  endif;
  
  return $return_string;
}
add_filter( 'woocommerce_get_price_html', 'bd_rrp_sale_price_html', 100, 2 );

WooCommerce: RRP in front of regular price

// Add text before regualr price only
function bd_rrp_price_html( $price, $product ) {

  $retun_string = 'RRP: ' . $price;
  return $return_string;
  
}
add_filter( 'woocommerce_get_price_html', 'bd_rrp_price_html', 100, 2 );

Under the RRP in front of regular price $retun_string should be $return_string. Works great, thanks.