We have a legacy tile server with tiles georeferenced to EPSG:4326 accessed as follows through openlayers:-

var OLDSERVICE =    new ol.layer.Tile({
                    title: 'OLD SERVICE',
                    type: 'base',
                    source: new ol.source.XYZ({
                                url: 'http://OLD_URL/maps/api/nls/{z}/{x}/{y}.jpg',
                                maxZoom: 15,
                                tilePixelRatio: 1})
                    });

The current base map is in EPSG:4326

We want to reproject these tiles to work on a base map with projection EPSG:27700

How would we do this?

I don't want to warp the tiles on the fly, would prefer to reprocess them for 27000. I am not very familiar with XYZ tile services as most of our stuff is done via a WMS service.