Manipulating data point in chart.js external tooltip

I’m using Chart.js to display some financial data in a pie chart, along with external tooltips. The data in the tooltip is displayed currently like this:

“Invoiceable: 1,202.5”

What I want is to firstly round the number to 1,200, and then add a ‘£’ sign before the data point so it reads:

“Invoiceable: £1,200”

I’m struggling to work out how to do this in this code below. Can anyone help?

 tooltip: {
            // Disable the on-canvas tooltip
            enabled: false,

            external: function(context) {
                // Tooltip Element
                let tooltipEl = document.getElementById('chartjs-tooltip');

                // Create element on first render
                if (!tooltipEl) {
                    tooltipEl = document.createElement('div');
                    tooltipEl.id = 'chartjs-tooltip';
                    tooltipEl.innerHTML = '<table></table>';
                    document.body.appendChild(tooltipEl);
                }

                // Hide if no tooltip
                const tooltipModel = context.tooltip;
                if (tooltipModel.opacity === 0) {
                    tooltipEl.style.opacity = 0;
                    return;
                }

                // Set caret Position
                tooltipEl.classList.remove('above', 'below', 'no-transform');
                if (tooltipModel.yAlign) {
                    tooltipEl.classList.add(tooltipModel.yAlign);
                } else {
                    tooltipEl.classList.add('no-transform');
                }

                function getBody(bodyItem) {
                     return bodyItem.lines;
                }

                // Set Text
                if (tooltipModel.body) {

                    const titleLines = tooltipModel.title || [];
                    const bodyLines = tooltipModel.body.map(getBody);

                    let innerHtml = '<thead>';

                    titleLines.forEach(function(title) {
                        innerHtml += '<tr><th>' + title + '</th></tr>';
                    });
                    innerHtml += '</thead><tbody>';

                    bodyLines.forEach(function(body, i) {
                        innerHtml += '<tr><td><div class="tooltip">'+ body + '</div></td></tr>';
                    });
                    innerHtml += '</tbody>';

                    let tableRoot = tooltipEl.querySelector('table');
                    tableRoot.innerHTML = innerHtml;
                }

                const position = context.chart.canvas.getBoundingClientRect();
                const bodyFont = Chart.helpers.toFont(tooltipModel.options.bodyFont);

                // Display, position, and set styles for font
                tooltipEl.style.opacity = .8;
                tooltipEl.style.position = 'absolute';
                tooltipEl.style.left = position.left + window.pageXOffset + tooltipModel.caretX + 'px';
                tooltipEl.style.top = position.top + window.pageYOffset + tooltipModel.caretY + 'px';
                tooltipEl.style.font = bodyFont.string;
                tooltipEl.style.padding = tooltipModel.padding + 'px ' + tooltipModel.padding + 'px';
                tooltipEl.style.pointerEvents = 'none';
            }
        }

FTR – I copied the above code from chart.js, which made it work, but I don’t presume to understand it for a second!